### 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
bool only_dirty = false;
  FrameSubscriber::FrameCaptureCallback callback;
  if (args->Length() > 1) {
    if (!args->GetNext(&only_dirty)) {
      args->ThrowError();
      return;
    }
  }
  if (!args->GetNext(&callback)) {
    args->ThrowError();
    return;
  }
  frame_subscriber_ =
      std::make_unique<FrameSubscriber>(ontents(), callback, only_dirty);
}
void WebContents::EndFrameSubscription() {