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
SetupFrameRate(false);
  begin_frame_timer_->SetActive(enabled);
  if (software_output_device_) {
    software_output_device_->SetActive(enabled);
  }
}
void OffScreenRenderWidgetHostView::SetPaintCallback(
    const OnPaintCallback& callback) {
  callback_ = callback;
}
void OffScreenRenderWidgetHostView::OnPaint(
    const gfx::Rect& damage_rect,
    const gfx::Size& bitmap_size,
    void* bitmap_pixels) {
  TRACE_EVENT0("electron", "OffScreenRenderWidgetHostView::OnPaint");
  if (!callback_.is_null())
    callback_.Run(damage_rect, bitmap_size, bitmap_pixels);
}
void OffScreenRenderWidgetHostView::SetPainting(bool painting) {
  painting_ = painting;
  if (software_output_device_) {
    software_output_device_->SetActive(painting);
  }
}
bool OffScreenRenderWidgetHostView::IsPainting() const {
  return painting_;
}
void OffScreenRenderWidgetHostView::SetFrameRate(int frame_rate) {