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
base::WeakPtr<AtomCopyFrameGenerator> generator,
      std::unique_ptr<cc::SingleReleaseCallback> release_callback,
      const gfx::Rect& damage_rect,
      std::unique_ptr<SkBitmap> bitmap,
      std::unique_ptr<SkAutoLockPixels> bitmap_pixels_lock,
      bool result) {
    gpu::SyncToken sync_token;
    if (result) {
      display_compositor::GLHelper* gl_helper =
          content::ImageTransportFactory::GetInstance()->GetGLHelper();
      if (gl_helper)
        gl_helper->GenerateSyncToken(&sync_token);
    }
    const bool lost_resource = !sync_token.HasData();
    release_callback->Run(sync_token, lost_resource);
    if (generator) {
      generator->CopyFromCompositingSurfaceFinished(
          damage_rect, std::move(bitmap), std::move(bitmap_pixels_lock),
          result);
    } else {
      bitmap_pixels_lock.reset();
      bitmap.reset();
    }
  }
  void CopyFromCompositingSurfaceFinished(