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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
std::vector<int> result(2);
  gfx::Point pos = window_->GetPosition();
  result[0] = pos.x();
  result[1] = pos.y();
  return result;
}
#if defined(OS_WIN) || defined(OS_MACOSX)
void BrowserWindow::MoveTop() {
  window_->MoveTop();
}
#endif
void BrowserWindow::SetTitle(const std::string& title) {
  window_->SetTitle(title);
}
std::string BrowserWindow::GetTitle() {
  return window_->GetTitle();
}
void BrowserWindow::FlashFrame(bool flash) {
  window_->FlashFrame(flash);
}
void BrowserWindow::SetSkipTaskbar(bool skip) {
  window_->SetSkipTaskbar(skip);
}
void BrowserWindow::SetSimpleFullScreen(bool simple_fullscreen) {
  window_->SetSimpleFullScreen(simple_fullscreen);
}
bool BrowserWindow::IsSimpleFullScreen() {