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/api/atom_api_web_contents.h
int32_t level,
                           const base::string16& message,
                           int32_t line_no,
                           const base::string16& source_id) override;
  content::WebContents* OpenURLFromTab(
      content::WebContents* source,
      const content::OpenURLParams& params) override;
  void BeforeUnloadFired(content::WebContents* tab,
                         bool proceed,
                         bool* proceed_to_fire_unload) override;
  void MoveContents(content::WebContents* source,
                    const gfx::Rect& pos) override;
  void CloseContents(content::WebContents* source) override;
  void ActivateContents(content::WebContents* contents) override;
  void UpdateTargetURL(content::WebContents* source, const GURL& url) override;
  bool IsPopupOrPanel(const content::WebContents* source) const override;
  void HandleKeyboardEvent(
      content::WebContents* source,
      const content::NativeWebKeyboardEvent& event) override;
  void EnterFullscreenModeForTab(content::WebContents* source,
                                 const GURL& origin) override;
  void ExitFullscreenModeForTab(content::WebContents* source) override;
  void RendererUnresponsive(content::WebContents* source) override;
  void RendererResponsive(content::WebContents* source) override;
  bool HandleContextMenu(const content::ContextMenuParams& params) override;
  bool OnGoToEntryOffset(int offset) override;
  void FindReply(content::WebContents* web_contents,
                 int request_id,
                 int number_of_matches,
                 const gfx::Rect& selection_rect,