<!--
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
const std::vector<mate::PersistentDictionary>& items) {
  window_->SetTouchBar(items);
}
void BrowserWindow::RefreshTouchBarItem(const std::string& item_id) {
  window_->RefreshTouchBarItem(item_id);
}
void BrowserWindow::SetEscapeTouchBarItem(
    const mate::PersistentDictionary& item) {
  window_->SetEscapeTouchBarItem(item);
}
int32_t BrowserWindow::ID() const {
  return weak_map_id();
}
v8::Local<v8::Value> BrowserWindow::WebContents(v8::Isolate* isolate) {
  if (web_contents_.IsEmpty()) {
    return v8::Null(isolate);
  }
  return v8::Local<v8::Value>::New(isolate, web_contents_);
}
void BrowserWindow::RemoveFromParentChildWindows() {
  if (parent_window_.IsEmpty())
    return;
  mate::Handle<BrowserWindow> parent;
  if (!mate::ConvertFromV8(isolate(), GetParentWindow(), &parent)
    || parent.IsEmpty()) {
    return;
  }
  parent->child_windows_.Remove(ID());
}
std::unique_ptr<SkRegion> BrowserWindow::DraggableRegionsToSkRegion(