### 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
content::RenderFrameHost* sender,
                      electron::mojom::ElectronBrowser::InvokeCallback callback,
                      Args&&... args) {
    DCHECK_CURRENTLY_ON(content::BrowserThread::UI);
    v8::Isolate* isolate = JavascriptEnvironment::GetIsolate();
    v8::HandleScope handle_scope(isolate);
    v8::Local<v8::Object> wrapper;
    if (!GetWrapper(isolate).ToLocal(&wrapper))
      return false;
    v8::Local<v8::Object> event = gin_helper::internal::CreateNativeEvent(
        isolate, wrapper, sender, std::move(callback));
    return EmitCustomEvent(name, event, std::forward<Args>(args)...);
  }
  WebContents* embedder() { return embedder_; }
#if BUILDFLAG(ENABLE_ELECTRON_EXTENSIONS)
  extensions::ScriptExecutor* script_executor() {
    return script_executor_.get();
  }
#endif
  
  void SetOwnerWindow(NativeWindow* owner_window);
  void SetOwnerWindow(content::WebContents* web_contents,
                      NativeWindow* owner_window);
  
  content::WebContents* GetWebContents() const;
  
  content::WebContents* GetDevToolsWebContents() const;
  InspectableWebContents* inspectable_web_contents() const {