### 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
if (!owner_window_)
    return;
  SetHtmlApiFullscreen(false);
  if (native_fullscreen_) {
    
    
    
    source->GetRenderViewHost()->GetWidget()->SynchronizeVisualProperties();
  }
  exclusive_access_manager_->fullscreen_controller()->ExitFullscreenModeForTab(
      source);
}
void WebContents::RendererUnresponsive(
    content::WebContents* source,
    content::RenderWidgetHost* render_widget_host,
    base::RepeatingClosure hang_monitor_restarter) {
  Emit("unresponsive");
}
void WebContents::RendererResponsive(
    content::WebContents* source,
    content::RenderWidgetHost* render_widget_host) {
  Emit("responsive");
}
bool WebContents::HandleContextMenu(content::RenderFrameHost& render_frame_host,