### 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,
    std::unique_ptr<content::WebContents> new_contents,
    const GURL& target_url,
    WindowOpenDisposition disposition,
    const gfx::Rect& initial_rect,
    bool user_gesture,
    bool* was_blocked) {
  auto* tracker = ChildWebContentsTracker::FromWebContents(new_contents.get());
  DCHECK(tracker);
  v8::Isolate* isolate = JavascriptEnvironment::GetIsolate();
  v8::Locker locker(isolate);
  v8::HandleScope handle_scope(isolate);
  auto api_ontents =
      CreateAndTake(isolate, std::move(new_contents), Type::kBrowserWindow);
  
  
  
  
  auto* frame = api_ontents->MainFrame();
  if (frame) {
    api_ontents->HandleNewRenderFrame(frame);
  }
  if (Emit("-add-new-contents", api_ontents, disposition, user_gesture,
           initial_rect.x(), initial_rect.y(), initial_rect.width(),
           initial_rect.height(), tracker->url, tracker->frame_name,
           tracker->referrer, tracker->raw_features, tracker->body)) {
    api_ontents->Destroy();
  }
}
content::WebConbContents::OpenURLFromTab(