### 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
#endif
  
  
  void OnGetZoomLevel(content::RenderFrameHost* frame_host,
                      IPC::Message* reply_msg);
  void InitZoomController(content::WebContents* web_contents,
                          const gin_helper::Dictionary& options);
  
  bool CanOverscrollContent() override;
  std::unique_ptr<content::EyeDropper> OpenEyeDropper(
      content::RenderFrameHost* frame,
      content::EyeDropperListener* listener) override;
  void RunFileChooser(content::RenderFrameHost* render_frame_host,
                      scoped_refptr<content::FileSelectListener> listener,
                      const blink::mojom::FileChooserParams& params) override;
  void EnumerateDirectory(content::WebContents* web_contents,
                          scoped_refptr<content::FileSelectListener> listener,
                          const base::FilePath& path) override;
  
  Profile* GetProfile() override;
  bool IsFullscreen() const override;
  void EnterFullscreen(const GURL& url,
                       ExclusiveAccessBubbleType bubble_type,
                       const int64_t display_id) override;
  void ExitFullscreen() override;
  void UpdateExclusiveAccessExitBubbleContent(
      const GURL& url,
      ExclusiveAccessBubbleType bubble_type,
      ExclusiveAccessBubbleHideCallback bubble_first_hide_callback,
      bool force_update) override;