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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
content::RenderFrameHost* rfh) {
  bool handled = true;
  IPC_BEGIN_MESSAGE_MAP_WITH_PARAM(BrowserWindow, message, rfh)
    IPC_MESSAGE_HANDLER(AtomFrameHostMsg_UpdateDraggableRegions,
                        UpdateDraggableRegions)
    IPC_MESSAGE_UNHANDLED(handled = false)
  IPC_END_MESSAGE_MAP()
  return handled;
}
void BrowserWindow::OnCloseContents() {
  DCHECK(web_contents());
  
  v8::Locker locker(isolate());
  v8::HandleScope handle_scope(isolate());
  for (v8::Local<v8::Value> value : child_windows_.Values(isolate())) {
    mate::Handle<BrowserWindow> child;
    if (mate::ConvertFromV8(isolate(), value, &child) && !child.IsEmpty())
      child->window_->CloseImmediately();
  }
  
  
  
  
  
  window_->CloseImmediately();
  
  window_unresponsive_closure_.Cancel();
}
void BrowserWindow::OnRendererResponsive() {