### 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 gin_helper::Dictionary preferences) {
  v8::Isolate* isolate = JavascriptEnvironment::GetIsolate();
  v8::Locker locker(isolate);
  v8::HandleScope handle_scope(isolate);
  
  
  pending_child_web_preferences_.Reset(isolate, preferences.GetHandle());
}
content::WebConbContents::CreateCustomWebContents(
    content::RenderFrameHost* opener,
    content::SiteInstance* source_site_instance,
    bool is_new_browsing_instance,
    const GURL& opener_url,
    const std::string& frame_name,
    const GURL& target_url,
    const content::StoragePartitionId& partition_id,
    content::SessionStorageNamespace* session_storage_namespace) {
  return nullptr;
}
void WebContents::AddNewContents(