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
return gfx::NativeView();
}
gfx::NativeViewAccessible
OffScreenRenderWidgetHostView::GetNativeViewAccessible() {
  return gfx::NativeViewAccessible();
}
ui::TextInputClient* OffScreenRenderWidgetHostView::GetTextInputClient() {
  return nullptr;
}
void OffScreenRenderWidgetHostView::Focus() {
}
bool OffScreenRenderWidgetHostView::HasFocus() const {
  return false;
}
bool OffScreenRenderWidgetHostView::IsSurfaceAvailableForCopy() const {
  return delegated_frame_host_->CanCopyToBitmap();
}
void OffScreenRenderWidgetHostView::Show() {
  if (is_showing_)
    return;
  is_showing_ = true;
  if (render_widget_host_)
    render_widget_host_->WasShown(ui::LatencyInfo());
  delegated_frame_host_->SetCompositor(compositor_.get());
  delegated_frame_host_->WasShown(ui::LatencyInfo());
  compositor_->ScheduleFullRedraw();
}
void OffScreenRenderWidgetHostView::Hide() {