### 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 for a feature request that matches the one I want to file, without success.

### Problem Description
When working with BrowserViews, there's no way to listen to focus changes on either BrowserView itself or its WebContents.

### Proposed Solution
On BrowserWindow, there are `focus` and `blur` events. The same events need to be emitted on BrowserView/WebContents.
i.e. : 
```
browserView.on('focus', () => {
    // view is focused, do stuff
});
```
or  
```
browserView.webContents.on('focus', () => {
    // contents are focused, do stuff
});
```

### Alternatives Considered
I can't think of a good way of working around this lack of functionality
shell/browser/api/electron_api_web_contents.h
bool IsOffScreen() const;
#if BUILDFLAG(ENABLE_OSR)
  void OnPaint(const gfx::Rect& dirty_rect, const SkBitmap& bitmap);
  void StartPainting();
  void StopPainting();
  bool IsPainting() const;
  void SetFrameRate(int frame_rate);
  int GetFrameRate() const;
#endif
  void Invalidate();
  gfx::Size GetSizeForNewRenderView(content::WebContents*) override;
  
  void SetZoomLevel(double level);
  double GetZoomLevel() const;
  void SetZoomFactor(gin_helper::ErrorThrower thrower, double factor);
  double GetZoomFactor() const;
  
  void OnEnterFullscreenModeForTab(
      content::RenderFrameHost* requesting_frame,
      const blink::mojom::FullscreenOptions& options,
      bool allowed);
  
  void OnCreateWindow(const GURL& target_url,
                      const content::Referrer& referrer,
                      const std::string& frame_name,
                      WindowOpenDisposition disposition,
                      const std::string& features,
                      const scoped_refptr<network::ResourceRequestBody>& body);
  
  std::vector<base::FilePath> GetPreloadPaths() const;