### 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.cc
content::WebContents* ontents,
                                        extensions::mojom::ViewType view_type) {
  
  type_ = GetTypeFromViewType(view_type);
  if (type_ == Type::kRemote)
    return;
  if (type_ == Type::kBackgroundPage)
    
    
    
    
    Pin(isolate);
  
  Observe(ontents);
  InitWithWebContents(std::unique_ptr<content::WebContents>(ontents),
                      GetBrowserContext(), IsGuest());
  inspectable_ontents_->GetView()->SetDelegate(this);
}
#endif
void WebContents::InitWithWebContents(