### 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::NavigationController::LoadURLParams params(url);
  if (!options.Get("httpReferrer", &params.referrer)) {
    GURL http_referrer;
    if (options.Get("httpReferrer", &http_referrer))
      params.referrer =
          content::Referrer(http_referrer.GetAsReferrer(),
                            network::mojom::ReferrerPolicy::kDefault);
  }
  std::string user_agent;
  if (options.Get("userAgent", &user_agent))
    ontents()->SetUserAgentOverride(
        blink::UserAgentOverride::UserAgentOnly(user_agent), false);
  std::string extra_headers;
  if (options.Get("extraHeaders", &extra_headers))
    params.extra_headers = extra_headers;
  scoped_refptr<network::ResourceRequestBody> body;
  if (options.Get("postData", &body)) {
    params.post_data = body;
    params.load_type = content::NavigationController::LOAD_TYPE_HTTP_POST;
  }
  GURL base_url_for_data_url;
  if (options.Get("baseURLForDataURL", &base_url_for_data_url)) {
    params.base_url_for_data_url = base_url_for_data_url;
    params.load_type = content::NavigationController::LOAD_TYPE_DATA;
  }
  bool reload_ignoring_cache = false;
  if (options.Get("reloadIgnoringCache", &reload_ignoring_cache) &&
      reload_ignoring_cache) {
    params.reload_type = content::ReloadType::BYPASSING_CACHE;