### 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
Emit("close");
  auto* autofill_driver_factory =
      AutofillDriverFactory::FromWebContents(ontents());
  if (autofill_driver_factory) {
    autofill_driver_factory->CloseAllPopups();
  }
  for (ExtendedWebContentsObserver& observer : observers_)
    observer.OnCloseContents();
}
void WebContents::ActivateContents(content::WebContents* source) {
  for (ExtendedWebContentsObserver& observer : observers_)
    observer.OnActivateContents();
}
void WebContents::UpdateTargetURL(content::WebContents* source,
                                  const GURL& url) {
  Emit("update-target-url", url);
}
bool WebContents::HandleKeyboardEvent(
    content::WebContents* source,
    const content::NativeWebKeyboardEvent& event) {
  if (type_ == Type::kWebView && embedder_) {
    
    return embedder_->HandleKeyboardEvent(source, event);
  } else {
    return PlatformHandleKeyboardEvent(source, event);
  }
}
#if !defined(OS_MAC)
bool WebContents::PlatformHandleKeyboardEvent(