### 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::OpenURLParams& params) {
  auto weak_this = GetWeakPtr();
  if (params.disposition != WindowOpenDisposition::CURRENT_TAB) {
    Emit("-new-window", params.url, "", params.disposition, "", params.referrer,
         params.post_data);
    return nullptr;
  }
  if (!weak_this || !ontents())
    return nullptr;
  content::NavigationController::LoadURLParams load_url_params(params.url);
  load_url_params.referrer = params.referrer;
  load_url_params.transition_type = params.transition;
  load_url_params.extra_headers = params.extra_headers;
  load_url_params.should_replace_current_entry =
      params.should_replace_current_entry;
  load_url_params.is_renderer_initiated = params.is_renderer_initiated;
  load_url_params.started_from_context_menu = params.started_from_context_menu;
  load_url_params.initiator_origin = params.initiator_origin;
  load_url_params.source_site_instance = params.source_site_instance;
  load_url_params.frame_tree_node_id = params.frame_tree_node_id;