### 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* source_contents,
    int opener_render_process_id,
    int opener_render_frame_id,
    const content::mojom::CreateNewWindowParams& params,
    content::WebContents* new_contents) {
  ChildWebContentsTracker::CreateForWebContents(new_contents);
  auto* tracker = ChildWebContentsTracker::FromWebContents(new_contents);
  tracker->url = params.target_url;
  tracker->frame_name = params.frame_name;
  tracker->referrer = params.referrer.To<content::Referrer>();
  tracker->raw_features = params.raw_features;
  tracker->body = params.body;
  v8::Isolate* isolate = JavascriptEnvironment::GetIsolate();
  v8::Locker locker(isolate);
  v8::HandleScope handle_scope(isolate);
  gin_helper::Dictionary dict;
  gin::ConvertFromV8(isolate, pending_child_web_preferences_.Get(isolate),
                     &dict);
  pending_child_web_preferences_.Reset();
  
  
  
  
  new WebContentsPreferences(new_contents, dict);
}
bool WebContents::IsWebContentsCreationOverridden(