### 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
}
  for (ExtendedWebContentsObserver& observer : observers_)
    observer.OnPageTitleUpdated(final_title, explicit_set);
  Emit("page-title-updated", final_title, explicit_set);
}
void WebContents::DidUpdateFaviconURL(
    content::RenderFrameHost* render_frame_host,
    const std::vector<blink::mojom::FaviconURLPtr>& urls) {
  std::set<GURL> unique_urls;
  for (const auto& iter : urls) {
    if (iter->icon_type != blink::mojom::FaviconIconType::kFavicon)
      continue;
    const GURL& url = iter->icon_url;
    if (url.is_valid())
      unique_urls.insert(url);
  }
  Emit("page-favicon-updated", unique_urls);
}
void WebContents::DevToolsReloadPage() {
  Emit("devtools-reload-page");
}
void WebContents::DevToolsFocused() {
  Emit("devtools-focused");
}
void WebContents::DevToolsOpened() {
  v8::Isolate* isolate = JavascriptEnvironment::GetIsolate();
  v8::Locker locker(isolate);
  v8::HandleScope handle_scope(isolate);
  DCHECK(inspectable_ontents_);
  DCHECK(inspectable_ontents_->GetDevToolsWebContents());