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
public:
  AtomCopyFrameGenerator(int frame_rate_threshold_ms,
                        OffScreenRenderWidgetHostView* view)
    : frame_rate_threshold_ms_(frame_rate_threshold_ms),
      view_(view),
      frame_pending_(false),
      frame_in_progress_(false),
      frame_retry_count_(0),
      weak_ptr_factory_(this) {
    last_time_ = base::Time::Now();
  }
  void GenerateCopyFrame(