### 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
auto* web_preferences = WebContentsPreferences::From(ontents());
  DCHECK(web_preferences);
  web_preferences->SetIgnoreMenuShortcuts(ignore);
}
void WebContents::SetAudioMuted(bool muted) {
  ontents()->SetAudioMuted(muted);
}
bool WebContents::IsAudioMuted() {
  return ontents()->IsAudioMuted();
}
bool WebContents::IsCurrentlyAudible() {
  return ontents()->IsCurrentlyAudible();
}
#if BUILDFLAG(ENABLE_PRINTING)
void WebContents::OnGetDefaultPrinter(
    base::Value print_settings,
    printing::CompletionCallback print_callback,
    std::u16string device_name,
    bool silent,
     <error, default_printer>
    std::pair<std::string, std::u16string> info) {
   The content::WebContents might be already deleted at this point, and the
   PrintViewManagerElectron class does not do null check.
  if (!ontents()) {
    if (print_callback)
      std::move(print_callback).Run(false, "failed");
    return;
  }
  if (!info.first.empty()) {