### 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
const GURL& url,
                              int error_code) {
  bool is_main_frame = !render_frame_host->GetParent();
  int frame_process_id = render_frame_host->GetProcess()->GetID();
  int frame_routing_id = render_frame_host->GetRoutingID();
  Emit("did-fail-load", error_code, "", url, is_main_frame, frame_process_id,
       frame_routing_id);
}
void WebContents::DidStartLoading() {
  Emit("did-start-loading");
}
void WebContents::DidStopLoading() {
  auto* web_preferences = WebContentsPreferences::From(ontents());
  if (web_preferences && web_preferences->ShouldUsePreferredSizeMode())
    ontents()->GetRenderViewHost()->EnablePreferredSizeMode();
  Emit("did-stop-loading");
}
bool WebContents::EmitNavigationEvent(