### 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
load_url_params.redirect_chain = params.redirect_chain;
  load_url_params.has_user_gesture = params.user_gesture;
  load_url_params.blob_url_loader_factory = params.blob_url_loader_factory;
  load_url_params.href_translate = params.href_translate;
  load_url_params.reload_type = params.reload_type;
  if (params.post_data) {
    load_url_params.load_type =
        content::NavigationController::LOAD_TYPE_HTTP_POST;
    load_url_params.post_data = params.post_data;
  }
  source->GetController().LoadURLWithParams(load_url_params);
  return source;
}
void WebContents::BeforeUnloadFired(content::WebContents* tab,
                                    bool proceed,
                                    bool* proceed_to_fire_unload) {
  if (type_ == Type::kBrowserWindow || type_ == Type::kOffScreen ||
      type_ == Type::kBrowserView)
    *proceed_to_fire_unload = proceed;
  else
    *proceed_to_fire_unload = true;
  
  Emit("before-unload-fired", proceed);
}
void WebContents::SetContentsBounds(content::WebContents* source,
                                    const gfx::Rect& rect) {
  for (ExtendedWebContentsObserver& observer : observers_)
    observer.OnSetContentBounds(rect);
}
void WebContents::CloseContents(content::WebContents* source) {