### 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
InspectableWebContents* inspectable_web_contents) {
  return new InspectableWebContentsViewViews(inspectable_web_contents);
}
InspectableWebContentsViewViews::InspectableWebContentsViewViews(
    InspectableWebContents* inspectable_web_contents)
    : inspectable_web_contents_(inspectable_web_contents),
      devtools_web_view_(new views::WebView(nullptr)),
      title_(u"Developer Tools") {
  if (!inspectable_web_contents_->IsGuest() &&
      inspectable_web_contents_->GetWebContents()->GetNativeView()) {
    auto* contents_web_view = new views::WebView(nullptr);
    contents_web_view->SetWebContents(
        inspectable_web_contents_->GetWebContents());
    contents_web_view_ = contents_web_view;
  } else {
    contents_web_view_ = new views::Label(u"No content under offscreen mode");
  }
  devtools_web_view_->SetVisible(false);
  AddChildView(devtools_web_view_);
  AddChildView(contents_web_view_);
}
InspectableWebContentsViewViews::~InspectableWebContentsViewViews() {
  if (devtools_window_)
    inspectable_web_contents()->SaveDevToolsBounds(
        devtools_window_->GetWindowBoundsInScreen());
}
views::View* InspectableWebContentsViewViews::GetView() {
  return this;
}
views::View* InspectableWebContentsViewViews::GetWebView() {