### 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,
    const content::NativeWebKeyboardEvent& event) {
  if (event.GetType() == blink::WebInputEvent::Type::kRawKeyDown ||
      event.GetType() == blink::WebInputEvent::Type::kKeyUp) {
    bool prevent_default = Emit("before-input-event", event);
    if (prevent_default) {
      return content::KeyboardEventProcessingResult::HANDLED;
    }
  }
  return content::KeyboardEventProcessingResult::NOT_HANDLED;
}
void WebContents::ContentsZoomChange(bool zoom_in) {
  Emit("zoom-changed", zoom_in ? "in" : "out");
}
Profile* WebContents::GetProfile() {
  return nullptr;
}
bool WebContents::IsFullscreen() const {
  return owner_window_ && owner_window_->IsFullscreen();
}
void WebContents::EnterFullscreen(const GURL& url,
                                  ExclusiveAccessBubbleType bubble_type,
                                  const int64_t display_id) {}
void WebContents::ExitFullscreen() {}