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
const gfx::Size& bitmap_size,
                          void* bitmap_pixels) {
  v8::MaybeLocal<v8::Object> buffer = node::Buffer::New(
      isolate(), reinterpret_cast<char*>(bitmap_pixels), sizeof(bitmap_pixels));
  if (!buffer.IsEmpty())
    Emit("paint", dirty_rect, buffer.ToLocalChecked(), bitmap_size);
}
void WebContents::StartPainting() {
  if (!IsOffScreen())
    return;
  auto* osr_rwhv = static_cast<OffScreenRenderWidgetHostView*>(
      web_contents()->GetRenderWidgetHostView());
  if (osr_rwhv) {
    osr_rwhv->SetPainting(true);
    osr_rwhv->Show();
  }
}
void WebContents::StopPainting() {
  if (!IsOffScreen())
    return;
  auto* osr_rwhv = static_cast<OffScreenRenderWidgetHostView*>(
      web_contents()->GetRenderWidgetHostView());
  if (osr_rwhv) {
    osr_rwhv->SetPainting(false);
    osr_rwhv->Hide();
  }
}
bool WebContents::IsPainting() const {