### 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
#endif
   Don't focus content after subframe navigations.
  if (!navigation_handle->IsInMainFrame())
    return;
   Only focus for top-level contents.
  if (type_ != Type::kBrowserWindow)
    return;
  ontents()->SetInitialFocus();
}
void WebContents::DidFinishNavigation(
    content::NavigationHandle* navigation_handle) {
  if (owner_window_) {
    owner_window_->NotifyLayoutWindowControlsOverlay();
  }
  if (!navigation_handle->HasCommitted())
    return;
  bool is_main_frame = navigation_handle->IsInMainFrame();
  content::RenderFrameHost* frame_host =
      navigation_handle->GetRenderFrameHost();
  int frame_process_id = -1, frame_routing_id = -1;
  if (frame_host) {
    frame_process_id = frame_host->GetProcess()->GetID();
    frame_routing_id = frame_host->GetRoutingID();
  }
  if (!navigation_handle->IsErrorPage()) {
     FIXME: All the Emit() calls below could potentially result in |this|
     being destroyed (by JS listening for the event and calling
     webContents.destroy()).
    auto url = navigation_handle->GetURL();
    bool is_same_document = navigation_handle->IsSameDocument();