### 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
std::unique_ptr<FrameSubscriber> frame_subscriber_;
#if BUILDFLAG(ENABLE_ELECTRON_EXTENSIONS)
  std::unique_ptr<extensions::ScriptExecutor> script_executor_;
#endif
  
  WebContents* embedder_ = nullptr;
  
  bool attached_ = false;
  
  WebContentsZoomController* zoom_controller_ = nullptr;
  
  Type type_ = Type::kBrowserWindow;
  int32_t id_;
  
  uint32_t find_in_page_request_id_ = 0;
  
  bool background_throttling_ = true;
  
  bool enable_devtools_ = true;
  
  base::ObserverList<ExtendedWebContentsObserver> observers_;
  v8::Global<v8::Value> pending_child_web_preferences_;
  
  base::WeakPtr<NativeWindow> owner_window_;
  bool offscreen_ = false;
  
  bool html_fullscreen_ = false;
  
  bool native_fullscreen_ = false;
  scoped_refptr<DevToolsFileSystemIndexer> devtools_file_system_indexer_;