### 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,
    blink::mojom::ConsoleMessageLevel level,
    const std::u16string& message,
    int32_t line_no,
    const std::u16string& source_id) {
  return Emit("console-message", static_cast<int32_t>(level), message, line_no,
              source_id);
}
void WebContents::OnCreateWindow(
    const GURL& target_url,
    const content::Referrer& referrer,
    const std::string& frame_name,
    WindowOpenDisposition disposition,
    const std::string& features,
    const scoped_refptr<network::ResourceRequestBody>& body) {
  Emit("-new-window", target_url, frame_name, disposition, features, referrer,
       body);
}
void WebContents::WebContentsCreatedWithFullParams(