### 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
if (fullscreen == is_html_fullscreen())
    return;
  html_fullscreen_ = fullscreen;
   Notify renderer of the html fullscreen change.
  ontents()
      ->GetRenderViewHost()
      ->GetWidget()
      ->SynchronizeVisualProperties();
   The embedder WebContents is separated from the frame tree of webview, so
   we must manually sync their fullscreen states.
  if (embedder_)
    embedder_->SetHtmlApiFullscreen(fullscreen);
  if (fullscreen) {
    Emit("enter-html-full-screen");
    owner_window_->NotifyWindowEnterHtmlFullScreen();
  } else {
    Emit("leave-html-full-screen");
    owner_window_->NotifyWindowLeaveHtmlFullScreen();
  }
   Make sure all child webviews quit html fullscreen.
  if (!fullscreen && !IsGuest()) {
    auto* manager = WebViewManager::GetWebViewManager(ontents());
    manager->ForEachGuest(
        ontents(), base::BindRepeating([](content::WebContents* guest) {
          WebContents* api_ontents = WebContents::From(guest);
          api_ontents->SetHtmlApiFullscreen(false);
          return false;
        }));
  }