### Preflight Checklist

- [X] I have read the [Contributing Guidelines](https://github.com/electron/electron/blob/master/CONTRIBUTING.md) for this project.
- [X] I agree to follow the [Code of Conduct](https://github.com/electron/electron/blob/master/CODE_OF_CONDUCT.md) that this project adheres to.
- [X] I have searched the [issue tracker](https://www.github.com/electron/electron/issues) for a feature request that matches the one I want to file, without success.

### Electron Version

11.*,  12.*,   13.*

### What operating system are you using?

Windows

### Operating System Version

Windows 10

### What arch are you using?

x64

### Last Known Working Electron version

_No response_

### Expected Behavior

`setBackgroundColor()` should set the background to the browser view. 

### Actual Behavior

The content of the BrowserView is still transparent.

![image](https://user-images.githubusercontent.com/2891206/122588825-d7726100-d05f-11eb-93d2-935eb93801c8.png)


### Testcase Gist URL

https://gist.github.com/ddramone/712a2fde2757736c9dbf6c3e7e0aa18b

### Additional Information



### Additional unusual behavior:

I tried to remove the background color from the parent BrowserWindow.  As a result

* **BrowserView that is fully visible** in the screen _(right)_ picks up its own background. But it is blended through the renderer content. 

* Another **BrowserView that is partially visible** on the screen _(left)_ still doesn't pick up its own background.

see the screenshot:

![image](https://user-images.githubusercontent.com/2891206/122589135-2d470900-d060-11eb-8ef2-ec4aa53a240e.png)


--- 
_figma.com is the example that doesn't have the background color to the document body._
shell/browser/ui/views/inspectable_web_contents_view_views.cc
CloseDevTools();
  if (!docked) {
    devtools_window_ = std::make_unique<views::Widget>();
    devtools_window_web_view_ = new views::WebView(nullptr);
    devtools_window_delegate_ = new DevToolsWindowDelegate(
        this, devtools_window_web_view_, devtools_window_.get());
    views::Widget::InitParams params;
    params.ownership = views::Widget::InitParams::WIDGET_OWNS_NATIVE_WIDGET;
    params.delegate = devtools_window_delegate_;
    params.bounds = inspectable_web_contents()->GetDevToolsBounds();
#if defined(OS_LINUX)
    params.wm_role_name = "devtools";
    if (GetDelegate())
      GetDelegate()->GetDevToolsWindowWMClass(&params.wm_class_name,
                                              &params.wm_class_class);
#endif
    devtools_window_->Init(std::move(params));
    devtools_window_->UpdateWindowIcon();
  }
  ShowDevTools(activate);
}
void InspectableWebContentsViewViews::SetContentsResizingStrategy(
    const DevToolsContentsResizingStrategy& strategy) {
  strategy_.CopyFrom(strategy);
  Layout();
}
void InspectableWebContentsViewViews::SetTitle(const std::u16string& title) {