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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
const std::vector<DraggableRegion>& regions) {
  std::unique_ptr<SkRegion> sk_region(new SkRegion);
  for (const DraggableRegion& region : regions) {
    sk_region->op(
        region.bounds.x(),
        region.bounds.y(),
        region.bounds.right(),
        region.bounds.bottom(),
        region.draggable ? SkRegion::kUnion_Op : SkRegion::kDifference_Op);
  }
  return sk_region;
}
void BrowserWindow::ScheduleUnresponsiveEvent(int ms) {
  if (!window_unresponsive_closure_.IsCancelled())
    return;
  window_unresponsive_closure_.Reset(
      base::Bind(&BrowserWindow::NotifyWindowUnresponsive, GetWeakPtr()));
  base::ThreadTaskRunnerHandle::Get()->PostDelayedTask(
      FROM_HERE,
      window_unresponsive_closure_.callback(),
      base::TimeDelta::FromMilliseconds(ms));
}
void BrowserWindow::NotifyWindowUnresponsive() {