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<cc::CopyOutputResult> result) {
    if (result->IsEmpty() || result->size().IsEmpty() ||
        !view_->render_widget_host()) {
      OnCopyFrameCaptureFailure(damage_rect);
      return;
    }
    if (result->HasTexture()) {
      PrepareTextureCopyOutputResult(damage_rect, std::move(result));
      return;
    }
    DCHECK(result->HasBitmap());
    PrepareBitmapCopyOutputResult(damage_rect, std::move(result));
  }
  void PrepareTextureCopyOutputResult(