### 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
electron::api::WebContents::Type val) {
    using Type = electron::api::WebContents::Type;
    std::string type;
    switch (val) {
      case Type::kBackgroundPage:
        type = "backgroundPage";
        break;
      case Type::kBrowserWindow:
        type = "window";
        break;
      case Type::kBrowserView:
        type = "browserView";
        break;
      case Type::kRemote:
        type = "remote";
        break;
      case Type::kWebView:
        type = "webview";
        break;
      case Type::kOffScreen:
        type = "offscreen";
        break;
      default:
        break;
    }
    return gin::ConvertToV8(isolate, type);
  }
  static bool FromV8(v8::Isolate* isolate,