### 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
base::BindRepeating(&WebContents::OnPaint, base::Unretained(this)));
      params.view = view;
      params.delegate_view = view;
      ontents = content::WebContents::Create(params);
      view->SetWebContents(ontents.get());
    } else {
#endif
      ontents = content::WebContents::Create(params);
#if BUILDFLAG(ENABLE_OSR)
    }
  } else if (IsOffScreen()) {
    bool transparent = false;
    options.Get(options::kTransparent, &transparent);
    content::WebContents::CreateParams params(session->browser_context());
    auto* view = new OffScreenWebContentsView(
        transparent,
        base::BindRepeating(&WebContents::OnPaint, base::Unretained(this)));
    params.view = view;
    params.delegate_view = view;
    ontents = content::WebContents::Create(params);
    view->SetWebContents(ontents.get());
#endif
  } else {
    content::WebContents::CreateParams params(session->browser_context());
    params.initially_hidden = !initially_shown;
    ontents = content::WebContents::Create(params);
  }
  InitWithSessionAndOptions(isolate, std::move(ontents), session, options);
}
void WebContents::InitZoomController(content::WebContents* ontents,