### 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
void SetAudioMuted(bool muted);
  bool IsAudioMuted();
  bool IsCurrentlyAudible();
  void SetEmbedder(const WebContents* embedder);
  void SetDevToolsWebContents(const WebContents* devtools);
  v8::Local<v8::Value> GetNativeView(v8::Isolate* isolate) const;
  void IncrementCapturerCount(gin::Arguments* args);
  void DecrementCapturerCount(gin::Arguments* args);
  bool IsBeingCaptured();
  void HandleNewRenderFrame(content::RenderFrameHost* render_frame_host);
#if BUILDFLAG(ENABLE_PRINTING)
  void OnGetDefaultPrinter(base::Value print_settings,
                           printing::CompletionCallback print_callback,
                           std::u16string device_name,
                           bool silent,
                           
                           std::pair<std::string, std::u16string> info);
  void Print(gin::Arguments* args);
  
  v8::Local<v8::Promise> PrintToPDF(base::DictionaryValue settings);
#endif
  void SetNextChildWebPreferences(const gin_helper::Dictionary);
  
  void AddWorkSpace(gin::Arguments* args, const base::FilePath& path);
  void RemoveWorkSpace(gin::Arguments* args, const base::FilePath& path);
  
  void Undo();
  void Redo();
  void Cut();
  void Copy();