### 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& file_system_path) {
  devtools_indexing_jobs_.erase(request_id);
  base::Value request_id_value(request_id);
  base::Value file_system_path_value(file_system_path);
  inspectable_ontents_->CallClientFunction(
      "DevToolsAPI.indexingDone", &request_id_value, &file_system_path_value,
      nullptr);
}
void WebContents::OnDevToolsSearchCompleted(
    int request_id,
    const std::string& file_system_path,
    const std::vector<std::string>& file_paths) {
  base::ListValue file_paths_value;
  for (const auto& file_path : file_paths) {
    file_paths_value.Append(file_path);
  }
  base::Value request_id_value(request_id);
  base::Value file_system_path_value(file_system_path);
  inspectable_ontents_->CallClientFunction(
      "DevToolsAPI.searchCompleted", &request_id_value, &file_system_path_value,
      &file_paths_value);
}
void WebContents::SetHtmlApiFullscreen(bool enter_fullscreen) {