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
static mate::Handle<WebContents> CreateFrom(
      v8::Isolate* isolate, content::WebContents* web_contents);
  
  static mate::Handle<WebContents> Create(
      v8::Isolate* isolate, const mate::Dictionary& options);
  static void BuildPrototype(v8::Isolate* isolate,
                             v8::Local<v8::FunctionTemplate> prototype);
  int GetID() const;
  Type GetType() const;
  bool Equal(const WebContents* web_contents) const;
  void LoadURL(const GURL& url, const mate::Dictionary& options);
  void DownloadURL(const GURL& url);
  GURL GetURL() const;
  base::string16 GetTitle() const;
  bool IsLoading() const;
  bool IsLoadingMainFrame() const;
  bool IsWaitingForResponse() const;
  void Stop();
  void ReloadIgnoringCache();
  void GoBack();
  void GoForward();
  void GoToOffset(int offset);
  bool IsCrashed() const;
  void SetUserAgent(const std::string& user_agent, mate::Arguments* args);
  std::string GetUserAgent();
  void InsertCSS(const std::string& css);
  bool SavePage(const base::FilePath& full_file_path,
                const content::SavePageType& save_type,
                const SavePageHandler::SavePageCallback& callback);
  void OpenDevTools(mate::Arguments* args);