### 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
v8::Isolate* isolate) const {
  auto* web_preferences = WebContentsPreferences::From(ontents());
  if (!web_preferences)
    return v8::Null(isolate);
  return gin::ConvertToV8(isolate, *web_preferences->last_preference());
}
v8::Local<v8::Value> WebContents::GetOwnerBrowserWindow(
    v8::Isolate* isolate) const {
  if (owner_window())
    return BrowserWindow::From(isolate, owner_window());
  else
    return v8::Null(isolate);
}
v8::Local<v8::Value> WebContents::Session(v8::Isolate* isolate) {
  return v8::Local<v8::Value>::New(isolate, session_);
}
content::WebConbContents::HostWebContents() const {
  if (!embedder_)
    return nullptr;
  return embedder_->ontents();
}
void WebContents::SetEmbedder(const WebContents* embedder) {