### 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
std::unique_ptr<ExclusiveAccessManager> exclusive_access_manager_;
  std::unique_ptr<DevToolsEyeDropper> eye_dropper_;
  ElectronBrowserContext* browser_context_;
  
  
  
  
  std::unique_ptr<InspectableWebContents> inspectable_web_contents_;
  
  typedef std::map<std::string, base::FilePath> PathsMap;
  PathsMap saved_files_;
  
  typedef std::
      map<int, scoped_refptr<DevToolsFileSystemIndexer::FileSystemIndexingJob>>
          DevToolsIndexingJobsMap;
  DevToolsIndexingJobsMap devtools_indexing_jobs_;
  scoped_refptr<base::SequencedTaskRunner> file_task_runner_;
#if BUILDFLAG(ENABLE_PRINTING)
  scoped_refptr<base::TaskRunner> print_task_runner_;
#endif
  
  content::RenderFrameHost* fullscreen_frame_ = nullptr;
  
  DevicePermissionMap granted_devices_;
  base::WeakPtrFactory<WebContents> weak_factory_{this};
};
}  
}  
#endif