I tired with the offscreen mode and here are the problems I found:
- [x] <del>The `data` of `paint` event is empty when loading simple local pages.</del>
- [x] The size of `data` is wrong, current implement uses the size of pointer (`sizeof(bitmap_pixels)`).
- [x] Crashes when opening devtools for offscreen window.
- [x] We should probably show a dummy view indicating it is offscreen instead of provide NULL.
- [x] The offscreen window is affecting the rendering of other windows. Try running `./script/test.py -g offscreen` and then refreshing the window.

/cc @gerhardberger @brenca

atom/browser/api/atom_api_web_contents.cc
auto view = web_contents()->GetRenderWidgetHostView();
  return view && view->HasFocus();
}
#endif
void WebContents::TabTraverse(bool reverse) {
  web_contents()->FocusThroughTabTraversal(reverse);
}
bool WebContents::SendIPCMessage(bool all_frames,
                                 const base::string16& channel,
                                 const base::ListValue& args) {
  return Send(new AtomViewMsg_Message(routing_id(), all_frames, channel, args));
}
void WebContents::SendInputEvent(v8::Isolate* isolate,
                                 v8::Local<v8::Value> input_event) {
  const auto view = web_contents()->GetRenderWidgetHostView();
  if (!view)
    return;
  const auto host = view->GetRenderWidgetHost();
  if (!host)
    return;
  int type = mate::GetWebInputEventType(isolate, input_event);
  if (blink::WebInputEvent::isMouseEventType(type)) {
    blink::WebMouseEvent mouse_event;
    if (mate::ConvertFromV8(isolate, input_event, &mouse_event)) {
      host->ForwardMouseEvent(mouse_event);
      return;
    }
  } else if (blink::WebInputEvent::isKeyboardEventType(type)) {
    content::NativeWebKeyboardEvent keyboard_event;