### 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
frame_subscriber_.reset();
}
void WebContents::StartDrag(const gin_helper::Dictionary& item,
                            gin::Arguments* args) {
  base::FilePath file;
  std::vector<base::FilePath> files;
  if (!item.Get("files", &files) && item.Get("file", &file)) {
    files.push_back(file);
  }
  v8::Local<v8::Value> icon_value;
  if (!item.Get("icon", &icon_value)) {
    gin_helper::ErrorThrower(args->isolate())
        .ThrowError("'icon' parameter is required");
    return;
  }
  NativeImage* icon = nullptr;
  if (!NativeImage::TryConvertNativeImage(args->isolate(), icon_value, &icon) ||
      icon->image().IsEmpty()) {
    return;
  }
   Start dragging.
  if (!files.empty()) {
    base::CurrentThread::ScopedNestableTaskAllower allow;
    DragFileItems(files, icon->image(), ontents()->GetNativeView());
  } else {
    gin_helper::ErrorThrower(args->isolate())
        .ThrowError("Must specify either 'file' or 'files' option");
  }
}
v8::Local<v8::Promise> WebContents::CapturePage(gin::Arguments* args) {