### 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
if (devtools_ontents_.IsEmpty())
    return v8::Null(isolate);
  else
    return v8::Local<v8::Value>::New(isolate, devtools_ontents_);
}
v8::Local<v8::Value> WebContents::Debugger(v8::Isolate* isolate) {
  if (debugger_.IsEmpty()) {
    auto handle = electron::api::Debugger::Create(isolate, ontents());
    debugger_.Reset(isolate, handle.ToV8());
  }
  return v8::Local<v8::Value>::New(isolate, debugger_);
}
content::RenderFrameHost* WebContents::MainFrame() {
  return ontents()->GetMainFrame();
}
void WebContents::NotifyUserActivation() {
  content::RenderFrameHost* frame = ontents()->GetMainFrame();
  if (frame)
    frame->NotifyUserActivation(
        blink::mojom::UserActivationNotificationType::kInteraction);
}
void WebContents::SetImageAnimationPolicy(const std::string& new_policy) {
  auto* web_preferences = WebContentsPreferences::From(ontents());
  web_preferences->SetImageAnimationPolicy(new_policy);
  ontents()->OnWebPreferencesChanged();
}
v8::Local<v8::Promise> WebContents::GetProcessMemoryInfo(v8::Isolate* isolate) {