### 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
if (owner_window) {
    owner_window_ = owner_window->GetWeakPtr();
    NativeWindowRelay::CreateForWebContents(ontents,
                                            owner_window->GetWeakPtr());
  } else {
    owner_window_ = nullptr;
    ontents->RemoveUserData(NativeWindowRelay::UserDataKey());
  }
#if BUILDFLAG(ENABLE_OSR)
  auto* osr_wcv = GetOffScreenWebContentsView();
  if (osr_wcv)
    osr_wcv->SetNativeWindow(owner_window);
#endif
}
content::WebConbContents::GetWebContents() const {
  if (!inspectable_ontents_)
    return nullptr;
  return inspectable_ontents_->GetWebContents();
}
content::WebConbContents::GetDevToolsWebContents() const {
  if (!inspectable_ontents_)
    return nullptr;
  return inspectable_ontents_->GetDevToolsWebContents();
}
void WebContents::WebContentsDestroyed() {
   Clear the pointer stored in wrapper.
  if (GetAllWebContents().Lookup(id_))
    GetAllWebContents().Remove(id_);
  v8::Isolate* isolate = JavascriptEnvironment::GetIsolate();
  v8::HandleScope scope(isolate);