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 gfx::Rect& damage_rect) {
    pending_damage_rect_.Union(damage_rect);
    const bool force_frame = (++frame_retry_count_ <= kFrameRetryLimit);
    OnCopyFrameCaptureCompletion(force_frame);
  }
  void OnCopyFrameCaptureSuccess(
      const gfx::Rect& damage_rect,
      const SkBitmap& bitmap,
      std::unique_ptr<SkAutoLockPixels> bitmap_pixels_lock) {
    view_->OnPaint(damage_rect,
                   gfx::Size(bitmap.width(), bitmap.height()),
                   bitmap.getPixels());
    if (frame_retry_count_ > 0)
      frame_retry_count_ = 0;
    OnCopyFrameCaptureCompletion(false);
  }
  void OnCopyFrameCaptureCompletion(bool force_frame) {