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
if (frame_rate <= 0)
    frame_rate = 1;
  if (frame_rate > 60)
    frame_rate = 60;
  frame_rate_ = frame_rate;
  SetupFrameRate(true);
}
int OffScreenRenderWidgetHostView::GetFrameRate() const {
  return frame_rate_;
}
void OffScreenRenderWidgetHostView::SetupFrameRate(bool force) {
  if (!force && frame_rate_threshold_ms_ != 0)
    return;
  frame_rate_threshold_ms_ = 1000 / frame_rate_;
  compositor_->vsync_manager()->SetAuthoritativeVSyncInterval(
      base::TimeDelta::FromMilliseconds(frame_rate_threshold_ms_));
  if (copy_frame_generator_.get()) {
    copy_frame_generator_->set_frame_rate_threshold_ms(
        frame_rate_threshold_ms_);
  }
  if (begin_frame_timer_.get()) {
    begin_frame_timer_->SetFrameRateThresholdMs(frame_rate_threshold_ms_);
  } else {
    begin_frame_timer_.reset(new AtomBeginFrameTimer(
        frame_rate_threshold_ms_,
        base::Bind(&OffScreenRenderWidgetHostView::OnBeginFrameTimerTick,
                   weak_ptr_factory_.GetWeakPtr())));
  }
}
void OffScreenRenderWidgetHostView::ResizeRootLayer() {