### 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
const gfx::Rect& initial_rect,
                      bool user_gesture,
                      bool* was_blocked) override;
  content::WebContents* OpenURLFromTab(
      content::WebContents* source,
      const content::OpenURLParams& params) override;
  void BeforeUnloadFired(content::WebContents* tab,
                         bool proceed,
                         bool* proceed_to_fire_unload) override;
  void SetContentsBounds(content::WebContents* source,
                         const gfx::Rect& pos) override;
  void CloseContents(content::WebContents* source) override;
  void ActivateContents(content::WebContents* contents) override;
  void UpdateTargetURL(content::WebContents* source, const GURL& url) override;
  bool HandleKeyboardEvent(
      content::WebContents* source,
      const content::NativeWebKeyboardEvent& event) override;
  bool PlatformHandleKeyboardEvent(
      content::WebContents* source,
      const content::NativeWebKeyboardEvent& event);
  content::KeyboardEventProcessingResult PreHandleKeyboardEvent(
      content::WebContents* source,
      const content::NativeWebKeyboardEvent& event) override;
  void ContentsZoomChange(bool zoom_in) override;
  void EnterFullscreenModeForTab(
      content::RenderFrameHost* requesting_frame,
      const blink::mojom::FullscreenOptions& options) override;
  void ExitFullscreenModeForTab(content::WebContents* source) override;
  void RendererUnresponsive(
      content::WebContents* source,