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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
window_->SetMinimumSize(gfx::Size(width, height));
}
std::vector<int> BrowserWindow::GetMinimumSize() {
  std::vector<int> result(2);
  gfx::Size size = window_->GetMinimumSize();
  result[0] = size.width();
  result[1] = size.height();
  return result;
}
void BrowserWindow::SetMaximumSize(int width, int height) {
  window_->SetMaximumSize(gfx::Size(width, height));
}
std::vector<int> BrowserWindow::GetMaximumSize() {
  std::vector<int> result(2);
  gfx::Size size = window_->GetMaximumSize();
  result[0] = size.width();
  result[1] = size.height();
  return result;
}
void BrowserWindow::SetSheetOffset(double offsetY, mate::Arguments* args) {
  double offsetX = 0.0;
  args->GetNext(&offsetX);
  window_->SetSheetOffset(offsetX, offsetY);
}
void BrowserWindow::SetResizable(bool resizable) {