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
int active_match_ordinal,
                 bool final_update) override;
  bool CheckMediaAccessPermission(
      content::WebContents* web_contents,
      const GURL& security_origin,
      content::MediaStreamType type) override;
  void RequestMediaAccessPermission(
      content::WebContents* web_contents,
      const content::MediaStreamRequest& request,
      const content::MediaResponseCallback& callback) override;
  void RequestToLockMouse(
      content::WebContents* web_contents,
      bool user_gesture,
      bool last_unlocked_by_target) override;
  std::unique_ptr<content::BluetoothChooser> RunBluetoothChooser(
      content::RenderFrameHost* frame,
      const content::BluetoothChooser::EventHandler& handler) override;
  
  void BeforeUnloadFired(const base::TimeTicks& proceed_time) override;
  void RenderViewDeleted(content::RenderViewHost*) override;
  void RenderProcessGone(base::TerminationStatus status) override;
  void DocumentLoadedInFrame(
      content::RenderFrameHost* render_frame_host) override;
  void DidFinishLoad(content::RenderFrameHost* render_frame_host,
                     const GURL& validated_url) override;
  void DidFailLoad(content::RenderFrameHost* render_frame_host,
                   const GURL& validated_url,
                   int error_code,
                   const base::string16& error_description,
                   bool was_ignored_by_handler) override;