### 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
const gin_helper::Dictionary& options) {
  WebContentsZoomController::CreateForWebContents(ontents);
  zoom_controller_ = WebContentsZoomController::FromWebContents(ontents);
  double zoom_factor;
  if (options.Get(options::kZoomFactor, &zoom_factor))
    zoom_controller_->SetDefaultZoomFactor(zoom_factor);
}
void WebContents::InitWithSessionAndOptions(
    v8::Isolate* isolate,
    std::unique_ptr<content::WebContents> owned_ontents,
    gin::Handle<api::Session> session,
    const gin_helper::Dictionary& options) {
  Observe(owned_ontents.get());
  InitWithWebContents(std::move(owned_ontents), session->browser_context(),
                      IsGuest());
  inspectable_ontents_->GetView()->SetDelegate(this);
  auto* prefs = ontents()->GetMutableRendererPrefs();