### 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
blink::WebMouseWheelEvent mouse_wheel_event;
    if (gin::ConvertFromV8(isolate, input_event, &mouse_wheel_event)) {
      if (IsOffScreen()) {
#if BUILDFLAG(ENABLE_OSR)
        GetOffScreenRenderWidgetHostView()->SendMouseWheelEvent(
            mouse_wheel_event);
#endif
      } else {
         Chromium expects phase info in wheel events (and applies a
         DCHECK to verify it). See: https:crbug.com/756524.
        mouse_wheel_event.phase = blink::WebMouseWheelEvent::kPhaseBegan;
        mouse_wheel_event.dispatch_type =
            blink::WebInputEvent::DispatchType::kBlocking;
        rwh->ForwardWheelEvent(mouse_wheel_event);
         Send a synthetic wheel event with phaseEnded to finish scrolling.
        mouse_wheel_event.has_synthetic_phase = true;
        mouse_wheel_event.delta_x = 0;
        mouse_wheel_event.delta_y = 0;
        mouse_wheel_event.phase = blink::WebMouseWheelEvent::kPhaseEnded;
        mouse_wheel_event.dispatch_type =
            blink::WebInputEvent::DispatchType::kEventNonBlocking;
        rwh->ForwardWheelEvent(mouse_wheel_event);
      }
      return;
    }
  }
  isolate->ThrowException(
      v8::Exception::Error(gin::StringToV8(isolate, "Invalid event object")));
}
void WebContents::BeginFrameSubscription(gin::Arguments* args) {