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
}
void OffScreenRenderWidgetHostView::ImeCompositionRangeChanged(
  const gfx::Range &, const std::vector<gfx::Rect>&) {
}
gfx::Size OffScreenRenderWidgetHostView::GetPhysicalBackingSize() const {
  return size_;
}
gfx::Size OffScreenRenderWidgetHostView::GetRequestedRendererSize() const {
  return size_;
}
int OffScreenRenderWidgetHostView::
  DelegatedFrameHostGetGpuMemoryBufferClientId()
    const {
  return render_widget_host_->GetProcess()->GetID();
}
ui::Layer* OffScreenRenderWidgetHostView::DelegatedFrameHostGetLayer() const {
  return const_cast<ui::Layer*>(root_layer_.get());
}
bool OffScreenRenderWidgetHostView::DelegatedFrameHostIsVisible() const {
  return !render_widget_host_->is_hidden();
}
SkColor OffScreenRenderWidgetHostView::DelegatedFrameHostGetGutterColor(
    SkColor color) const {
  return color;
}
gfx::Size OffScreenRenderWidgetHostView::DelegatedFrameHostDesiredSizeInDIP()
  const {
  return size_;
}
bool OffScreenRenderWidgetHostView::DelegatedFrameCanCreateResizeLock() const {