### Issue Details

* **Electron Version:**
  * electron@11.1.1
* **Operating System:**
  * Windows-10-10.0.19041-SP0

### Expected Behavior
 Window with circular corners

### Actual Behavior
When the window is maximized and then minimized, the square and black borders appear.

### To Reproduce
Just put in the main.js 
``trasnparent = true,  frame = false
``
And with html and css create  the circular corners (Below I add the code)

### Screenshots
![pb](https://user-images.githubusercontent.com/56138728/104198981-e429c380-53f4-11eb-940a-2dbe485a83c4.jpg)

### Additional Information
#### Main.js:
```const { app, BrowserWindow } = require('electron')

let win;

function createWindow () {
  win = new BrowserWindow({
    width: 800,
    height: 600,
    minWidth:400,
    minHeight: 300,
    frame: false,
    transparent: true,
    webPreferences: {
      nodeIntegration: true
    }
  })

  win.loadFile('index.html')
}


app.whenReady().then(createWindow)

app.on('window-all-closed', () => {
  if (process.platform !== 'darwin') {
    app.quit()
  }
})

app.on('activate', () => {
  if (BrowserWindow.getAllWindows().length === 0) {
    createWindow()
  }
})
```
#### index.html:
```<!DOCTYPE html>
<html>

<head>
    <meta charset="UTF-8">
    <link rel="stylesheet" href="styles.css">
</head>

<body>
    <div class="Body">

        <div class="WindowBar">
            <div class="LogoTitleBar">
                <div class="LogoBar">
                    <img class="LogoBar"
                        src="https://i.pinimg.com/originals/46/0e/78/460e78cb4c61f55da3802ca5d1d68d15.jpg" alt="">
                </div>
                <div class="TitleBar">
                    My App
                </div>
            </div>
            <div class="MenuBar">
                <div class="btn btn1"></div>
                <div class="btn btn2"></div>
                <div class="btn btn3"></div>
            </div>
        </div>
        <div class="content">
            <!-- <img class="ImgContent" src="https://wallpaperaccess.com/full/3542858.png" alt=""> -->
        </div>


    </div>

</body>

</html>
```

#### styles.css:
```html,
body {
    width : 100%;
    height: 100vh;
    margin: 0;
}

:root {
    --radius_value: 15px;
    --radius_btn  : 15px;
}



.Body {
    width            : 100%;
    height           : 100vh;
    /* 100% of the viewport height */
    margin           : 0px;
    border-radius    : var(--radius_value);
    /* background    : #b42424; */
    /* background       : #212121; */
    display          : flex;
    flex-direction   : column;
    align-items      : stretch;
    background-image: url("https://wallpaperaccess.com/full/3542858.png");
    background-size: cover;
    /* background-size: 100% 100%; */
    /* opacity: 0.5; */
}

.WindowBar {
    display                      : flex;
    justify-content              : space-between;
    align-items                  : center;
    width                        : 100%;
    height                       : 30px;
    /* margin                    : 1px; */
    border-top-right-radius      : var(--radius_value);
    border-top-left-radius       : var(--radius_value);
    /* border-radius             : var(--radius_value); */
    background                   : #313131;
    -webkit-app-region           : drag;
    -webkit-user-select          : none;
    color                        : #fff;
    opacity: 0.98;
}

.LogoTitleBar {
    display        : flex;
    justify-content: space-between;
    align-items    : center;
    margin-left    : 10px;
}

.LogoBar {
    width           : var(--radius_btn);
    height          : var(--radius_btn);
    border-radius   : 50%;
    /* background   : #000; */
}

.TitleBar {
    margin-left    : 5px;
    display        : flex;
    align-items    : center;
    justify-content: center;
    font-family    : sans-serif;
    font-weight    : 100;
    font-size      : 13px;
}

.MenuBar {
    display            : flex;
    align-items        : center;
    -webkit-app-region : no-drag;
    -webkit-user-select: none;
    padding-right      : 10px;

}

.btn {
    display        : flex;
    justify-content: center;
    align-items    : center;
    margin         : 5px;
    width          : var(--radius_btn);
    height         : var(--radius_btn);
    border-radius  : 50%;
    cursor             : pointer;

}

.btn1 {
    background: orange;

}

.btn2 {
    background: green;
}

.btn3 {
    background: red;
}

.content {
    display                   : flex;
    align-items               : center;
    justify-content           : center;
    border-bottom-right-radius: var(--radius_value);
    border-bottom-left-radius : var(--radius_value);
    overflow                  : hidden;
}

.ImgContent {
    width  : 100%;
    height : 100vh;
}
```

shell/browser/native_window_views_win.cc
case WM_ENDSESSION: {
      if (w_param) {
        NotifyWindowEndSession();
      }
      return false;
    }
    case WM_PARENTNOTIFY: {
      if (LOWORD(w_param) == WM_CREATE) {
        
        
        
        
        
        if (!legacy_window_) {
          legacy_window_ = reinterpret_cast<HWND>(l_param);
        }
      }
      return false;
    }
    case WM_CONTEXTMENU: {
      bool prevent_default = false;
      NotifyWindowSystemContextMenu(GET_X_LPARAM(l_param),
                                    GET_Y_LPARAM(l_param), &prevent_default);
      return prevent_default;
    }
    default:
      return false;
  }
}
void NativeWindowViews::HandleSizeEvent(WPARAM w_param, LPARAM l_param) {