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
if (!is_showing_)
    return;
  if (render_widget_host_)
    render_widget_host_->WasHidden();
  delegated_frame_host_->WasHidden();
  delegated_frame_host_->ResetCompositor();
  is_showing_ = false;
}
bool OffScreenRenderWidgetHostView::IsShowing() {
  return is_showing_;
}
gfx::Rect OffScreenRenderWidgetHostView::GetViewBounds() const {
  return gfx::Rect(size_);
}
void OffScreenRenderWidgetHostView::SetBackgroundColor(SkColor color) {
  if (transparent_)
    color = SkColorSetARGB(SK_AlphaTRANSPARENT, 0, 0, 0);
  content::RenderWidgetHostViewBase::SetBackgroundColor(color);
  const bool opaque = !transparent_ && GetBackgroundOpaque();
  if (render_widget_host_)
    render_widget_host_->SetBackgroundOpaque(opaque);
}
gfx::Size OffScreenRenderWidgetHostView::GetVisibleViewportSize() const {
  return size_;
}
void OffScreenRenderWidgetHostView::SetInsets(const gfx::Insets& insets) {
}
bool OffScreenRenderWidgetHostView::LockMouse() {