<!--
Thanks for opening an issue! A few things to keep in mind:

- The issue tracker is only for bugs and feature requests.
- Before reporting a bug, please try reproducing your issue against
  the latest version of Electron.
- If you need general advice, join our Slack: http://atom-slack.herokuapp.com
-->

* Electron version: 1.8.4
* Operating system: Windows

### Expected behavior

Off screen windows should respond to the `.focus()` method being called by making the content appear to be focused.

### Actual behavior

Currently the undocumented method `focusOnWebView()` method must be called in order to "focus" an offscreen window.

This behaviour should either be documented [in the off-screen rendering guide](https://electronjs.org/docs/tutorial/offscreen-rendering), or the `.focus()` method should fix this

### How to reproduce

* Create an offscreen window, and have its content output to some location (In my case an OpenGL texture)
* Go to a web page, such as google
* Observe no caret in focused text box, and selected text is the wrong colour

Not sure if this is just a documentation issue or a bug... Feel free to resolve as if it were working as intended and document this issue in the off-screen rendering guide.

atom/browser/api/atom_api_browser_window.cc
auto* existing_preferences =
        WebContentsPreferences::From(web_contents->web_contents());
    base::DictionaryValue web_preferences_dict;
    if (mate::ConvertFromV8(isolate, web_preferences.GetHandle(),
                            &web_preferences_dict)) {
      existing_preferences->dict()->Clear();
      existing_preferences->Merge(web_preferences_dict);
    }
  } else {
    
    web_contents = WebContents::Create(isolate, web_preferences);
  }
  Init(isolate, wrapper, options, web_contents);
}
void BrowserWindow::Init(v8::Isolate* isolate,
                         v8::Local<v8::Object> wrapper,
                         const mate::Dictionary& options,
                         mate::Handle<class WebContents> web_contents) {
  web_contents_.Reset(isolate, web_contents.ToV8());
  api_web_contents_ = web_contents.get();
  api_web_contents_->AddObserver(this);
  Observe(api_web_contents_->web_contents());
  
  mate::Dictionary(isolate, web_contents->GetWrapper()).Set(
      "browserWindowOptions", options);
  
  mate::Handle<BrowserWindow> parent;