### 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
v8::Local<v8::Value> GetLastWebPreferences(v8::Isolate* isolate) const;
  
  v8::Local<v8::Value> GetOwnerBrowserWindow(v8::Isolate* isolate) const;
  
  void NotifyUserActivation();
  v8::Local<v8::Promise> TakeHeapSnapshot(v8::Isolate* isolate,
                                          const base::FilePath& file_path);
  v8::Local<v8::Promise> GetProcessMemoryInfo(v8::Isolate* isolate);
  
  int32_t ID() const { return id_; }
  v8::Local<v8::Value> Session(v8::Isolate* isolate);
  content::WebContents* HostWebContents() const;
  v8::Local<v8::Value> DevToolsWebContents(v8::Isolate* isolate);
  v8::Local<v8::Value> Debugger(v8::Isolate* isolate);
  content::RenderFrameHost* MainFrame();
  WebContentsZoomController* GetZoomController() { return zoom_controller_; }
  void AddObserver(ExtendedWebContentsObserver* obs) {
    observers_.AddObserver(obs);
  }
  void RemoveObserver(ExtendedWebContentsObserver* obs) {
    
    if (!observers_.empty())
      observers_.RemoveObserver(obs);
  }
  bool EmitNavigationEvent(const std::string& event,
                           content::NavigationHandle* navigation_handle);
  
  template <typename... Args>
  bool EmitWithSender(base::StringPiece name,