### 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 std::string& channel,
                            blink::CloneableMessage arguments) {
  TRACE_EVENT1("electron", "WebContents::MessageTo", "channel", channel);
  auto* target_ontents = FromID(ontents_id);
  if (target_ontents) {
    content::RenderFrameHost* frame = target_ontents->MainFrame();
    DCHECK(frame);
    v8::HandleScope handle_scope(JavascriptEnvironment::GetIsolate());
    gin::HFrameMain> web_frame_main =
        WebFrameMain::From(JavascriptEnvironment::GetIsolate(), frame);
    if (!web_frame_main->CheckRenderFrame())
      return;
    int32_t sender_id = ID();
    web_frame_main->GetRendererApi()->Message(false /* interl,
                                              std::move(arguments), sender_id);
  }
}
void WebContents::MessageHost(const std::string& channel,
                              blink::CloneableMessage arguments,
                              content::RenderFrameHost* render_frame_host) {