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.h
#else
class CALayer;
class NSWindow;
#endif
#endif
namespace atom {
class AtomCopyFrameGenerator;
class AtomBeginFrameTimer;
class OffScreenRenderWidgetHostView
    : public content::RenderWidgetHostViewBase,
#if defined(OS_MACOSX)
      public ui::AcceleratedWidgetMacNSView,
#endif
      public ui::CompositorDelegate,
      public content::DelegatedFrameHostClient {
 public:
  OffScreenRenderWidgetHostView(bool transparent,
                                content::RenderWidgetHost* render_widget_host,
                                NativeWindow* native_window);
  ~OffScreenRenderWidgetHostView() override;
  
  bool OnMessageReceived(const IPC::Message&) override;
  void InitAsChild(gfx::NativeView) override;
  content::RenderWidgetHost* GetRenderWidgetHost(void) const override;
  void SetSize(const gfx::Size &) override;
  void SetBounds(const gfx::Rect &) override;
  gfx::Vector2dF GetLastScrollOffset(void) const override;
  gfx::NativeView GetNativeView(void) const override;
  gfx::NativeViewAccessible GetNativeViewAccessible(void) override;
  ui::TextInputClient* GetTextInputClient() override;