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
void CloseDevTools();
  bool IsDevToolsOpened();
  bool IsDevToolsFocused();
  void ToggleDevTools();
  void EnableDeviceEmulation(const blink::WebDeviceEmulationParams& params);
  void DisableDeviceEmulation();
  void InspectElement(int x, int y);
  void InspectServiceWorker();
  void HasServiceWorker(const base::Callback<void(bool)>&);
  void UnregisterServiceWorker(const base::Callback<void(bool)>&);
  void SetAudioMuted(bool muted);
  bool IsAudioMuted();
  void Print(mate::Arguments* args);
  
  void PrintToPDF(const base::DictionaryValue& setting,
                  const PrintToPDFCallback& callback);
  
  void AddWorkSpace(mate::Arguments* args, const base::FilePath& path);
  void RemoveWorkSpace(mate::Arguments* args, const base::FilePath& path);
  
  void Undo();
  void Redo();
  void Cut();
  void Copy();
  void Paste();
  void PasteAndMatchStyle();
  void Delete();
  void SelectAll();
  void Unselect();
  void Replace(const base::string16& word);