### 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
std::unique_ptr<content::WebContents> ontents,
    ElectronBrowserContext* browser_context,
    bool is_guest) {
  browser_context_ = browser_context;
  ontents->SetDelegate(this);
#if BUILDFLAG(ENABLE_PRINTING)
  PrintPreviewMessageHandler::CreateForWebContents(ontents.get());
  PrintViewManagerElectron::CreateForWebContents(ontents.get());
  printing::CreateCompositeClientIfNeeded(ontents.get(),
                                          browser_context->GetUserAgent());
#endif
#if BUILDFLAG(ENABLE_PDF_VIEWER)
  pdf::PDFWebContentsHelper::CreateForWebContentsWithClient(
      ontents.get(),
      std::make_unique<ElectronPDFWebContentsHelperClient>());
#endif
  
  auto* web_preferences = WebContentsPreferences::From(ontents.get());
  offscreen_ = web_preferences && web_preferences->IsOffscreen();
  
  inspectable_ontents_ = std::make_unique<InspectableWebContents>(
      std::move(ontents), browser_context->prefs(), is_guest);
  inspectable_ontents_->SetDelegate(this);
}
WebContents::~WebContents() {