### 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
auto* isolate = JavascriptEnvironment::GetIsolate();
  v8::HandleScope scope(isolate);
  v8::Lobject> wrapper;
  if (!GetWrapper(isolate).ToLocal(&wrapper))
    return;
  delete this;
}
void WebContents::Destroy() {
  
  
  if (Browser::Get()->is_shutting_down() || IsGuest()) {
    DeleteThisIfAlive();
  } else {
    base::PostTask(FROM_HERE, {content::BrowserThread::UI},
                   base::BindOnce(
                       [](base::WeakPtr<WebContents> contents) {
                         if (contents)
                           contents->DeleteThisIfAlive();
                       },
                       GetWeakPtr()));
  }
}
bool WebContents::DidAddMessageToConsole(