### 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
const content::WebContents* source) {
  return html_fullscreen_;
}
bool WebContents::TakeFocus(content::WebContents* source, bool reverse) {
  if (source && source->GetOutermostWebContents() == source) {
     If this is the outermost web contents and the user has tabbed or
     shift + tabbed through all the elements, reset the focus back to
     the first or last element so that it doesn't stay in the body.
    source->FocusThroughTabTraversal(reverse);
    return true;
  }
  return false;
}
content::PictureInPictureResult WebContents::EnterPictureInPicture(
    content::WebContents* ontents,
    const viz::SurfaceId& surface_id,
    const gfx::Size& natural_size) {
#if BUILDFLAG(ENABLE_PICTURE_IN_PICTURE)
  return PictureInPictureWindowManager::GetInstance()->EnterPictureInPicture(
      ontents, surface_id, natural_size);
#else
  return content::PictureInPictureResult::kNotSupported;
#endif
}
void WebContents::ExitPictureInPicture() {
#if BUILDFLAG(ENABLE_PICTURE_IN_PICTURE)
  PictureInPictureWindowManager::GetInstance()->ExitPictureInPicture();
#endif
}
void WebContents::DevToolsSaveToFile(const std::string& url,