### 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
WindowOpenDisposition val) {
    std::string disposition = "other";
    switch (val) {
      case WindowOpenDisposition::CURRENT_TAB:
        disposition = "default";
        break;
      case WindowOpenDisposition::NEW_FOREGROUND_TAB:
        disposition = "foreground-tab";
        break;
      case WindowOpenDisposition::NEW_BACKGROUND_TAB:
        disposition = "background-tab";
        break;
      case WindowOpenDisposition::NEW_POPUP:
      case WindowOpenDisposition::NEW_WINDOW:
        disposition = "new-window";
        break;
      case WindowOpenDisposition::SAVE_TO_DISK:
        disposition = "save-to-disk";
        break;
      default:
        break;
    }
    return gin::ConvertToV8(isolate, disposition);
  }
};
template <>
struct Converter<content::SavePageType> {
  static bool FromV8(v8::Isolate* isolate,