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
return ++request_id_;
  }
  
  void OnCursorChange(const content::WebCursor& cursor);
  
  void OnRendererMessage(const base::string16& channel,
                         const base::ListValue& args);
  
  void OnRendererMessageSync(const base::string16& channel,
                             const base::ListValue& args,
                             IPC::Message* message);
  v8::Global<v8::Value> session_;
  v8::Global<v8::Value> devtools_web_contents_;
  v8::Global<v8::Value> debugger_;
  std::unique_ptr<WebViewGuestDelegate> guest_delegate_;
  
  WebContents* embedder_;
  
  Type type_;
  
  uint32_t request_id_;
  
  bool background_throttling_;
  DISALLOW_COPY_AND_ASSIGN(WebContents);
};
}  
}  
#endif