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
if (!IsOffScreen())
    return false;
  const auto* osr_rwhv = static_cast<OffScreenRenderWidgetHostView*>(
      web_contents()->GetRenderWidgetHostView());
  return osr_rwhv && osr_rwhv->IsPainting();
}
void WebContents::SetFrameRate(int frame_rate) {
  if (!IsOffScreen())
    return;
  auto* osr_rwhv = static_cast<OffScreenRenderWidgetHostView*>(
      web_contents()->GetRenderWidgetHostView());
  if (osr_rwhv)
    osr_rwhv->SetFrameRate(frame_rate);
}
int WebContents::GetFrameRate() const {
  if (!IsOffScreen())
    return 0;
  const auto* osr_rwhv = static_cast<OffScreenRenderWidgetHostView*>(
      web_contents()->GetRenderWidgetHostView());
  return osr_rwhv ? osr_rwhv->GetFrameRate() : 0;
}
v8::Local<v8::Value> WebContents::GetWebPreferences(v8::Isolate* isolate) {
  WebContentsPreferences* web_preferences =
      WebContentsPreferences::FromWebContents(web_contents());
  return mate::ConvertToV8(isolate, *web_preferences->web_preferences());
}
v8::Local<v8::Value> WebContents::GetOwnerBrowserWindow() {