### 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
return contents_web_view_;
}
void InspectableWebContentsViewViews::ShowDevTools(bool activate) {
  if (devtools_visible_)
    return;
  devtools_visible_ = true;
  if (devtools_window_) {
    devtools_window_web_view_->SetWebContents(
        inspectable_web_contents_->GetDevToolsWebContents());
    devtools_window_->SetBounds(
        inspectable_web_contents()->GetDevToolsBounds());
    if (activate) {
      devtools_window_->Show();
    } else {
      devtools_window_->ShowInactive();
    }
    
    if (GetDelegate())
      GetDelegate()->DevToolsResized();
  } else {
    devtools_web_view_->SetVisible(true);
    devtools_web_view_->SetWebContents(
        inspectable_web_contents_->GetDevToolsWebContents());
    devtools_web_view_->RequestFocus();
    Layout();
  }
}
void InspectableWebContentsViewViews::CloseDevTools() {