### 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
v8::Local<v8::Value> input_event) {
  content::RenderWidgetHostView* view =
      ontents()->GetRenderWidgetHostView();
  if (!view)
    return;
  content::RenderWidgetHost* rwh = view->GetRenderWidgetHost();
  blink::WebInputEvent::Type type =
      gin::GetWebInputEventType(isolate, input_event);
  if (blink::WebInputEvent::IsMouseEventType(type)) {
    blink::WebMouseEvent mouse_event;
    if (gin::ConvertFromV8(isolate, input_event, &mouse_event)) {
      if (IsOffScreen()) {
#if BUILDFLAG(ENABLE_OSR)
        GetOffScreenRenderWidgetHostView()->SendMouseEvent(mouse_event);
#endif
      } else {
        rwh->ForwardMouseEvent(mouse_event);
      }
      return;
    }
  } else if (blink::WebInputEvent::IsKeyboardEventType(type)) {
    content::NativeWebKeyboardEvent keyboard_event(
        blink::WebKeyboardEvent::Type::kRawKeyDown,
        blink::WebInputEvent::Modifiers::kNoModifiers, ui::EventTimeForNow());
    if (gin::ConvertFromV8(isolate, input_event, &keyboard_event)) {
      rwh->ForwardKeyboardEvent(keyboard_event);
      return;
    }
  } else if (type == blink::WebInputEvent::Type::kMouseWheel) {