### 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 (IsOffScreen()) {
#if BUILDFLAG(ENABLE_OSR)
    auto* osr_rwhv = GetOffScreenRenderWidgetHostView();
    if (osr_rwhv)
      osr_rwhv->Invalidate();
#endif
  } else {
    auto* const window = owner_window();
    if (window)
      window->Invalidate();
  }
}
gfx::Size WebContents::GetSizeForNewRenderView(content::WebContents* wc) {
  if (IsOffScreen() && wc == ontents()) {
    auto* relay = NativeWindowRelay::FromWebContents(ontents());
    if (relay) {
      auto* owner_window = relay->GetNativeWindow();
      return owner_window ? owner_window->GetSize() : gfx::Size();
    }
  }
  return gfx::Size();
}
void WebContents::SetZoomLevel(double level) {
  zoom_controller_->SetZoomLevel(level);
}
double WebContents::GetZoomLevel() const {
  return zoom_controller_->GetZoomLevel();
}
void WebContents::SetZoomFactor(gin_helper::ErrorThrower thrower,