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/osr/osr_render_widget_host_view.cc
const base::TimeTicks& timebase, const base::TimeDelta& interval) {
  render_widget_host_->UpdateVSyncParameters(timebase, interval);
}
void OffScreenRenderWidgetHostView::SetBeginFrameSource(
    cc::BeginFrameSource* source) {
}
std::unique_ptr<cc::SoftwareOutputDevice>
  OffScreenRenderWidgetHostView::CreateSoftwareOutputDevice(
    ui::Compositor* compositor) {
  DCHECK_EQ(compositor_.get(), compositor);
  DCHECK(!copy_frame_generator_);
  DCHECK(!software_output_device_);
  software_output_device_ = new OffScreenOutputDevice(transparent_,
      base::Bind(&OffScreenRenderWidgetHostView::OnPaint,
                 weak_ptr_factory_.GetWeakPtr()));
  return base::WrapUnique(software_output_device_);
}
bool OffScreenRenderWidgetHostView::InstallTransparency() {
  if (transparent_) {
    SetBackgroundColor(SkColor());
    compositor_->SetHostHasTransparentBackground(true);
    return true;
  }
  return false;
}
bool OffScreenRenderWidgetHostView::IsAutoResizeEnabled() const {
  return false;
}
void OffScreenRenderWidgetHostView::OnSetNeedsBeginFrames(bool enabled) {