### 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
Window is already in fullscreen mode, save the state.
  if (enter_fullscreen && owner_window_->IsFullscreen()) {
    native_fullscreen_ = true;
    UpdateHtmlApiFullscreen(true);
    return;
  }
   Exit html fullscreen state but not window's fullscreen mode.
  if (!enter_fullscreen && native_fullscreen_) {
    UpdateHtmlApiFullscreen(false);
    return;
  }
   Set fullscreen on window if allowed.
  auto* web_preferences = WebContentsPreferences::From(GetWebContents());
  bool html_fullscreenable =
      web_preferences
          ? !web_preferences->ShouldDisableHtmlFullscreenWindowResize()
          : true;
  if (html_fullscreenable) {
    owner_window_->SetFullScreen(enter_fullscreen);
  }
  UpdateHtmlApiFullscreen(enter_fullscreen);
  native_fullscreen_ = false;
}
void WebContents::UpdateHtmlApiFullscreen(bool fullscreen) {