### 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
const gfx::Size& pref_size) {
  Emit("preferred-size-changed", pref_size);
}
bool WebContents::CanOverscrollContent() {
  return false;
}
std::unique_ptr<content::EyeDropper> WebContents::OpenEyeDropper(
    content::RenderFrameHost* frame,
    content::EyeDropperListener* listener) {
  return ShowEyeDropper(frame, listener);
}
void WebContents::RunFileChooser(
    content::RenderFrameHost* render_frame_host,
    scoped_refptr<content::FileSelectListener> listener,
    const blink::mojom::FileChooserParams& params) {
  FileSelectHelper::RunFileChooser(render_frame_host, std::move(listener),
                                   params);
}
void WebContents::EnumerateDirectory(
    content::WebContents* ontents,
    scoped_refptr<content::FileSelectListener> listener,
    const base::FilePath& path) {
  FileSelectHelper::EnumerateDirectory(ontents, std::move(listener), path);
}
bool WebContents::IsFullscreenForTabOrPending(