### 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
TRACE_EVENT1("electron", "WebContents::MessageHost", "channel", channel);
   webContents.emit('ipc-message-host', new Event(), channel, args);
  EmitWithSender("ipc-message-host", render_frame_host,
                 electron::mojom::ElectronBrowser::InvokeCallback(), channel,
                 std::move(arguments));
}
void WebContents::UpdateDraggableRegions(
    std::vector<mojom::DraggableRegionPtr> regions) {
  for (ExtendedWebContentsObserver& observer : observers_)
    observer.OnDraggableRegionsUpdated(regions);
}
void WebContents::DidStartNavigation(
    content::NavigationHandle* navigation_handle) {
  EmitNavigationEvent("did-start-navigation", navigation_handle);
}
void WebContents::DidRedirectNavigation(
    content::NavigationHandle* navigation_handle) {
  EmitNavigationEvent("did-redirect-navigation", navigation_handle);
}
void WebContents::ReadyToCommitNavigation(
    content::NavigationHandle* navigation_handle) {
   Don't focus content in an inactive window.
  if (!owner_window())
    return;
#if defined(OS_MAC)
  if (!owner_window()->IsActive())
    return;
#else
  if (!owner_window()->widget()->IsActive())
    return;