### 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 base::TimeTicks& proceed_time) {
  
  
}
void WebContents::HandleNewRenderFrame(
    content::RenderFrameHost* render_frame_host) {
  auto* rwhv = render_frame_host->GetView();
  if (!rwhv)
    return;
  
  auto* web_preferences = WebContentsPreferences::From(ontents());
  if (web_preferences) {
    bool guest = IsGuest() || type_ == Type::kBrowserView;
    absl::optional<SkColor> color =
        guest ? SK_ColorTRANSPARENT : web_preferences->GetBackgroundColor();
    ontents()->SetPageBaseBackgroundColor(color);
    SetBackgroundColor(rwhv, color.value_or(SK_ColorWHITE));
  }
  if (!background_throttling_)
    render_frame_host->GetRenderViewHost()->SetSchedulerThrottling(false);
  auto* rwh_impl =
      static_cast<content::RenderWidgetHostImpl*>(rwhv->GetRenderWidgetHost());
  if (rwh_impl)
    rwh_impl->disable_hidden_ = !background_throttling_;
  auto* web_frame = WebFrameMain::FromRenderFrameHost(render_frame_host);
  if (web_frame)
    web_frame->Connect();
}
void WebContents::OnBackgroundColorChanged() {