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);
  const float orgScaleFactor = scale_factor_;
  const bool scaleFactorDidChange = (orgScaleFactor != scale_factor_);
  gfx::Size size = GetViewBounds().size();
  if (!scaleFactorDidChange && size == root_layer_->bounds().size())
    return;
  const gfx::Size& size_in_pixels =
      gfx::ConvertSizeToPixel(scale_factor_, size);
  root_layer_->SetBounds(gfx::Rect(size));
  compositor_->SetScaleAndSize(scale_factor_, size_in_pixels);
}
}