### 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
WebContents& operator=(const WebContents&) = delete;
 private:
  
  WebContents(v8::Isolate* isolate, content::WebContents* web_contents);
  
  WebContents(v8::Isolate* isolate,
              std::unique_ptr<content::WebContents> web_contents,
              Type type);
  
  WebContents(v8::Isolate* isolate, const gin_helper::Dictionary& options);
  ~WebContents() override;
  
  void DeleteThisIfAlive();
  
  
  void InitWithWebContents(std::unique_ptr<content::WebContents> web_contents,
                           ElectronBrowserContext* browser_context,
                           bool is_guest);
  void InitWithSessionAndOptions(
      v8::Isolate* isolate,
      std::unique_ptr<content::WebContents> web_contents,
      gin::Handle<class Session> session,
      const gin_helper::Dictionary& options);
#if BUILDFLAG(ENABLE_ELECTRON_EXTENSIONS)
  void InitWithExtensionView(v8::Isolate* isolate,
                             content::WebContents* web_contents,
                             extensions::mojom::ViewType view_type);
#endif