### 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.h
return inspectable_web_contents_.get();
  }
  NativeWindow* owner_window() const { return owner_window_.get(); }
  bool is_html_fullscreen() const { return html_fullscreen_; }
  void set_fullscreen_frame(content::RenderFrameHost* rfh) {
    fullscreen_frame_ = rfh;
  }
  
  void Message(bool internal,
               const std::string& channel,
               blink::CloneableMessage arguments,
               content::RenderFrameHost* render_frame_host);
  void Invoke(bool internal,
              const std::string& channel,
              blink::CloneableMessage arguments,
              electron::mojom::ElectronBrowser::InvokeCallback callback,
              content::RenderFrameHost* render_frame_host);
  void OnFirstNonEmptyLayout(content::RenderFrameHost* render_frame_host);
  void ReceivePostMessage(const std::string& channel,
                          blink::TransferableMessage message,
                          content::RenderFrameHost* render_frame_host);
  void MessageSync(
      bool internal,