### 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
bool initially_shown = type_ != Type::kBrowserView;
  options.Get(options::kShow, &initially_shown);
  
  std::string partition;
  gin::Handle<api::Session> session;
  if (options.Get("session", &session) && !session.IsEmpty()) {
  } else if (options.Get("partition", &partition)) {
    session = Session::FromPartition(isolate, partition);
  } else {
    
    session = Session::FromPartition(isolate, "");
  }
  session_.Reset(isolate, session.ToV8());
  std::unique_ptr<content::WebContents> ontents;
  if (IsGuest()) {
    scoped_refptr<content::SiteInstance> site_instance =
        content::SiteInstance::CreateForURL(session->browser_context(),
                                            GURL("chrome-guest:fake-host"));
    content::WebContents::CreateParams params(session->browser_context(),
                                              site_instance);
    guest_delegate_ =
        std::make_unique<WebViewGuestDelegate>(embedder_->ontents(), this);
    params.guest_delegate = guest_delegate_.get();
#if BUILDFLAG(ENABLE_OSR)
    if (embedder_ && embedder_->IsOffScreen()) {
      auto* view = new OffScreenWebContentsView(
          false,