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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
mate::Dictionary web_preferences = mate::Dictionary::CreateEmpty(isolate);
  options.Get(options::kWebPreferences, &web_preferences);
  
  v8::Local<v8::Value> value;
  if (options.Get(options::kBackgroundColor, &value))
    web_preferences.Set(options::kBackgroundColor, value);
  v8::Local<v8::Value> transparent;
  if (options.Get("transparent", &transparent))
    web_preferences.Set("transparent", transparent);
#if defined(ENABLE_OSR)
  
  bool offscreen;
  if (web_preferences.Get("offscreen", &offscreen) && offscreen) {
    auto window_options = const_cast<mate::Dictionary&>(options);
    window_options.Set(options::kFrame, false);
  }
#endif
  if (options.Get("webContents", &web_contents) && !web_contents.IsEmpty()) {