### 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
gin::HContents> WebContents::CreateFromWebPreferences(
    v8::Isolate* isolate,
    const gin_helper::Dictionary& web_preferences) {
   Check if webPreferences has |webContents| option.
  gin::HContents> ontents;
  if (web_preferences.GetHidden("webContents", &ontents) &&
      !ontents.IsEmpty()) {
     Set webPreferences from options if using an existing webContents.
     These preferences will be used when the webContent launches new
     render processes.
    auto* existing_preferences =
        WebContentsPreferences::From(ontents->ontents());
    gin_helper::Dictionary web_preferences_dict;
    if (gin::ConvertFromV8(isolate, web_preferences.GetHandle(),
                           &web_preferences_dict)) {
      existing_preferences->SetFromDictionary(web_preferences_dict);
      absl::optional<SkColor> color =
          existing_preferences->GetBackgroundColor();
      ontents->ontents()->SetPageBaseBackgroundColor(color);