### 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
ontents()->GetController().Reload(content::ReloadType::BYPASSING_CACHE,
                                         /* check_for_repost */ true);
}
void WebContents::DownloadURL(const GURL& url) {
  auto* browser_context = ontents()->GetBrowserContext();
  auto* download_manager = browser_context->GetDownloadManager();
  std::unique_ptr<download::DownloadUrlParameters> download_params(
      content::DownloadRequestUtils::CreateDownloadForWebContentsMainFrame(
          ontents(), url, MISSING_TRAFFIC_ANNOTATION));
  download_manager->DownloadUrl(std::move(download_params));
}
std::u16string WebContents::GetTitle() const {