### 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
void WebContents::UpdateExclusiveAccessExitBubbleContent(
    const GURL& url,
    ExclusiveAccessBubbleType bubble_type,
    ExclusiveAccessBubbleHideCallback bubble_first_hide_callback,
    bool force_update) {}
void WebContents::OnExclusiveAccessUserInput() {}
content::WebConbContents::GetActiveWebContents() {
  return ontents();
}
bool WebContents::CanUserExitFullscreen() const {
  return true;
}
bool WebContents::IsExclusiveAccessBubbleDisplayed() const {
  return false;
}
void WebContents::EnterFullscreenModeForTab(
    content::RenderFrameHost* requesting_frame,
    const blink::mojom::FullscreenOptions& options) {
  auto* source = content::WebContents::FromRenderFrameHost(requesting_frame);
  auto* permission_helper =
      WebContentsPermissionHelper::FromWebContents(source);
  auto callback =
      base::BindRepeating(&WebContents::OnEnterFullscreenModeForTab,
                          base::Unretained(this), requesting_frame, options);
  permission_helper->RequestFullscreenPermission(callback);
  exclusive_access_manager_->fullscreen_controller()->EnterFullscreenModeForTab(
      requesting_frame, options.display_id);
}
void WebContents::OnEnterFullscreenModeForTab(