### 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
Emit("paint", dirty_rect, gfx::Image::CreateFrom1xBitmap(bitmap));
}
void WebContents::StartPainting() {
  auto* osr_wcv = GetOffScreenWebContentsView();
  if (osr_wcv)
    osr_wcv->SetPainting(true);
}
void WebContents::StopPainting() {
  auto* osr_wcv = GetOffScreenWebContentsView();
  if (osr_wcv)
    osr_wcv->SetPainting(false);
}
bool WebContents::IsPainting() const {
  auto* osr_wcv = GetOffScreenWebContentsView();
  return osr_wcv && osr_wcv->IsPainting();
}
void WebContents::SetFrameRate(int frame_rate) {
  auto* osr_wcv = GetOffScreenWebContentsView();
  if (osr_wcv)
    osr_wcv->SetFrameRate(frame_rate);
}
int WebContents::GetFrameRate() const {
  auto* osr_wcv = GetOffScreenWebContentsView();
  return osr_wcv ? osr_wcv->GetFrameRate() : 0;
}
#endif
void WebContents::Invalidate() {