### 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
content::WebContents* source,
    const content::NativeWebKeyboardEvent& event) {
  
  if (event.windows_key_code == ui::VKEY_ESCAPE && is_html_fullscreen()) {
    ExitFullscreenModeForTab(source);
    return true;
  }
  
  auto* web_preferences = WebContentsPreferences::From(source);
  if (web_preferences && web_preferences->ShouldIgnoreMenuShortcuts())
    return false;
  
  if (owner_window()) {
    owner_window()->HandleKeyboardEvent(source, event);
    return true;
  }
  return false;
}
#endif
content::KeyboardEventProcessingResult WebContents::PreHandleKeyboardEvent(