### 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
bool esc_key_locked) {
  exclusive_access_manager_->keyboard_lock_controller()->RequestKeyboardLock(
      ontents, esc_key_locked);
}
void WebContents::CancelKeyboardLockRequest(
    content::WebContents* ontents) {
  exclusive_access_manager_->keyboard_lock_controller()
      ->CancelKeyboardLockRequest(ontents);
}
bool WebContents::CheckMediaAccessPermission(
    content::RenderFrameHost* render_frame_host,
    const GURL& security_origin,
    blink::mojom::MediaStreamType type) {
  auto* ontents =
      content::WebContents::FromRenderFrameHost(render_frame_host);
  auto* permission_helper =
      WebContentsPermissionHelper::FromWebContents(ontents);
  return permission_helper->CheckMediaAccessPermission(security_origin, type);
}
void WebContents::RequestMediaAccessPermission(