* Output of `node_modules/.bin/electron --version`: 4.0.0-beta.8
* Operating System (Platform and Version): Windows 10 1803
* Output of `node_modules/.bin/electron --version` on last known working Electron version (if applicable): 4.0.0-beta.7

**Expected Behavior**
`BrowserWindow.getNativeWindowHandle()` should return different value for different window

**Actual behavior**
`BrowserWindow.getNativeWindowHandle()` returns same value for different window, which is not true nativeWindowHandle for any of them.

**To Reproduce**
Open two `BrowserWindow`, and run `getNativeWindowHandle` on both `BrowserWindow`
```js
const { remote } = require('electron')
w1 = new remote.BrowserWindow()
w2 = new remote.BrowserWindow()
console.log(w1.getNativeWindowHandle(), w2.getNativeWindowHandle())
```
![image](https://user-images.githubusercontent.com/8194131/49328509-7411a800-f5b5-11e8-9a24-827aa589d937.png)


atom/browser/native_window_views.cc
constraints.set_minimum_size(min_size);
    }
    if (constraints.HasMaximumSize()) {
      gfx::Size max_size = constraints.GetMaximumSize();
      max_size.set_height(max_size.height() + menu_bar_height);
      constraints.set_maximum_size(max_size);
    }
    SetContentSizeConstraints(constraints);
    
    SetContentSize(content_size);
  }
}
void NativeWindowViews::SetBrowserView(NativeBrowserView* view) {
  if (!content_view())
    return;
  if (browser_view()) {
    content_view()->RemoveChildView(
        browser_view()->GetInspectableWebContentsView()->GetView());
    set_browser_view(nullptr);
  }
  if (!view) {
    return;
  }
  
  
  set_browser_view(view);
  content_view()->AddChildView(
      view->GetInspectableWebContentsView()->GetView());
}
void NativeWindowViews::SetParentWindow(NativeWindow* parent) {