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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
if (!Browser::Get()->is_ready()) {
    args->ThrowError("Cannot create BrowserWindow before app is ready");
    return nullptr;
  }
  if (args->Length() > 1) {
    args->ThrowError();
    return nullptr;
  }
  mate::Dictionary options;
  if (!(args->Length() == 1 && args->GetNext(&options))) {
    options = mate::Dictionary::CreateEmpty(args->isolate());
  }
  return new BrowserWindow(args->isolate(), args->GetThis(), options);
}
void BrowserWindow::Close() {
  window_->Close();
}
void BrowserWindow::Focus() {
  window_->Focus(true);
}
void BrowserWindow::Blur() {
  window_->Focus(false);
}
bool BrowserWindow::IsFocused() {
  return window_->IsFocused();
}
void BrowserWindow::Show() {