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* 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
return window_->IsSimpleFullScreen();
}
void BrowserWindow::SetKiosk(bool kiosk) {
  window_->SetKiosk(kiosk);
}
bool BrowserWindow::IsKiosk() {
  return window_->IsKiosk();
}
void BrowserWindow::SetBackgroundColor(const std::string& color_name) {
  SkColor color = ParseHexColor(color_name);
  window_->SetBackgroundColor(color);
  auto* view = web_contents()->GetRenderWidgetHostView();
  if (view)
    view->SetBackgroundColor(color);
}
void BrowserWindow::SetHasShadow(bool has_shadow) {
  window_->SetHasShadow(has_shadow);
}
bool BrowserWindow::HasShadow() {
  return window_->HasShadow();
}
void BrowserWindow::SetOpacity(const double opacity) {
  window_->SetOpacity(opacity);
}
double BrowserWindow::GetOpacity() {
  return window_->GetOpacity();
}
void BrowserWindow::FocusOnWebView() {