### 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* requesting_frame,
    const blink::mojom::FullscreenOptions& options,
    bool allowed) {
  if (!allowed || !owner_window_)
    return;
  auto* source = content::WebContents::FromRenderFrameHost(requesting_frame);
  if (IsFullscreenForTabOrPending(source)) {
    DCHECK_EQ(fullscreen_frame_, source->GetFocusedFrame());
    return;
  }
  SetHtmlApiFullscreen(true);
  if (native_fullscreen_) {
    
    
    source->GetRenderViewHost()->GetWidget()->SynchronizeVisualProperties();
  }
}
void WebContents::ExitFullscreenModeForTab(content::WebContents* source) {