### 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.h
kBackgroundPage,  
    kBrowserWindow,   
    kBrowserView,     
    kRemote,          
    kWebView,         
    kOffScreen,       
  };
  
  static gin::Handle<WebContents> New(v8::Isolate* isolate,
                                      const gin_helper::Dictionary& options);
  
  
  
  static gin::Handle<WebContents> CreateAndTake(
      v8::Isolate* isolate,
      std::unique_ptr<content::WebContents> web_contents,
      Type type);
  
  
  static WebContents* From(content::WebContents* web_contents);
  static WebContents* FromID(int32_t id);
  
  
  
  
  static gin::Handle<WebContents> FromOrCreate(
      v8::Isolate* isolate,
      content::WebContents* web_contents);
  static gin::Handle<WebContents> CreateFromWebPreferences(