### 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
content::WebContents* ontents)
    : content::WebContentsObserver(ontents),
      type_(Type::kRemote),
      id_(GetAllWebContents().Add(this)),
      devtools_file_system_indexer_(
          base::MakeRefCounted<DevToolsFileSystemIndexer>()),
      exclusive_access_manager_(std::make_unique<ExclusiveAccessManager>(this)),
      file_task_runner_(
          base::ThreadPool::CreateSequencedTaskRunner({base::MayBlock()}))
#if BUILDFLAG(ENABLE_PRINTING)
      ,
      print_task_runner_(CreatePrinterHandlerTaskRunner())
#endif
{
#if BUILDFLAG(ENABLE_ELECTRON_EXTENSIONS)
  
  extensions::mojom::ViewType view_type = extensions::GetViewType(ontents);
  if (view_type != extensions::mojom::ViewType::kInvalid) {
    InitWithExtensionView(isolate, ontents, view_type);
  }
  extensions::ElectronExtensionWebContentsObserver::CreateForWebContents(
      ontents);