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,
      std::unique_ptr<SkBitmap> bitmap,
      std::unique_ptr<SkAutoLockPixels> bitmap_pixels_lock,
      bool result) {
    DCHECK(!bitmap_);
    bitmap_ = std::move(bitmap);
    if (result) {
      OnCopyFrameCaptureSuccess(damage_rect, *bitmap_,
                                std::move(bitmap_pixels_lock));
    } else {
      bitmap_pixels_lock.reset();
      OnCopyFrameCaptureFailure(damage_rect);
    }
  }
  void PrepareBitmapCopyOutputResult(
      const gfx::Rect& damage_rect,
      std::unique_ptr<cc::CopyOutputResult> result) {
    DCHECK(result->HasBitmap());
    std::unique_ptr<SkBitmap> source = result->TakeBitmap();
    DCHECK(source);
    if (source) {
      std::unique_ptr<SkAutoLockPixels> bitmap_pixels_lock(
          new SkAutoLockPixels(*source));
      OnCopyFrameCaptureSuccess(damage_rect, *source,
                                std::move(bitmap_pixels_lock));
    } else {
      OnCopyFrameCaptureFailure(damage_rect);
    }
  }
  void OnCopyFrameCaptureFailure(