### 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::RenderFrameHost* render_frame_host) {
  HandleNewRenderFrame(render_frame_host);
  
  
  
  
  
  auto* rfh_impl =
      static_cast<content::RenderFrameHostImpl*>(render_frame_host);
  if (rfh_impl->lifecycle_state() ==
      content::RenderFrameHostImpl::LifecycleStateImpl::kSpeculative) {
    return;
  }
  content::RenderFrameHost::LifecycleState lifecycle_state =
      render_frame_host->GetLifecycleState();
  if (lifecycle_state == content::RenderFrameHost::LifecycleState::kActive) {
    v8::Isolate* isolate = JavascriptEnvironment::GetIsolate();
    v8::HandleScope handle_scope(isolate);
    gin_helper::Dictionary details =
        gin_helper::Dictionary::CreateEmpty(isolate);
    details.SetGetter("frame", render_frame_host);
    Emit("frame-created", details);
  }
}
void WebContents::RenderFrameDeleted(