### 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
bool CanGoToOffset(int offset) const;
  void GoToOffset(int offset);
  bool CanGoToIndex(int index) const;
  void GoToIndex(int index);
  int GetActiveIndex() const;
  void ClearHistory();
  int GetHistoryLength() const;
  const std::string GetWebRTCIPHandlingPolicy() const;
  void SetWebRTCIPHandlingPolicy(const std::string& webrtc_ip_handling_policy);
  std::string GetMediaSourceID(content::WebContents* request_web_contents);
  bool IsCrashed() const;
  void ForcefullyCrashRenderer();
  void SetUserAgent(const std::string& user_agent);
  std::string GetUserAgent();
  void InsertCSS(const std::string& css);
  v8::Local<v8::Promise> SavePage(const base::FilePath& full_file_path,
                                  const content::SavePageType& save_type);
  void OpenDevTools(gin::Arguments* args);
  void CloseDevTools();
  bool IsDevToolsOpened();
  bool IsDevToolsFocused();
  void ToggleDevTools();
  void EnableDeviceEmulation(const blink::DeviceEmulationParams& params);
  void DisableDeviceEmulation();
  void InspectElement(int x, int y);
  void InspectSharedWorker();
  void InspectSharedWorkerById(const std::string& workerId);
  std::vector<scoped_refptr<content::DevToolsAgentHost>> GetAllSharedWorkers();
  void InspectServiceWorker();
  void SetIgnoreMenuShortcuts(bool ignore);