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
void Focus(void) override;
  bool HasFocus(void) const override;
  bool IsSurfaceAvailableForCopy(void) const override;
  void Show(void) override;
  void Hide(void) override;
  bool IsShowing(void) override;
  gfx::Rect GetViewBounds(void) const override;
  gfx::Size GetVisibleViewportSize() const override;
  void SetInsets(const gfx::Insets&) override;
  void SetBackgroundColor(SkColor color) override;
  bool LockMouse(void) override;
  void UnlockMouse(void) override;
  bool GetScreenColorProfile(std::vector<char>*) override;
#if defined(OS_MACOSX)
  ui::AcceleratedWidgetMac* GetAcceleratedWidgetMac() const override;
  void SetActive(bool active) override;
  void ShowDefinitionForSelection() override;
  bool SupportsSpeech() const override;
  void SpeakSelection() override;
  bool IsSpeaking() const override;
  void StopSpeaking() override;
#endif  
  
  void OnSwapCompositorFrame(uint32_t, std::unique_ptr<cc::CompositorFrame>)
    override;
  void ClearCompositorFrame(void) override;
  void InitAsPopup(content::RenderWidgetHostView *rwhv, const gfx::Rect& rect)
    override;
  void InitAsFullscreen(content::RenderWidgetHostView *) override;
  void UpdateCursor(const content::WebCursor &) override;