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.cc
atom::api::WebContents::Type val) {
    using Type = atom::api::WebContents::Type;
    std::string type = "";
    switch (val) {
      case Type::BACKGROUND_PAGE: type = "backgroundPage"; break;
      case Type::BROWSER_WINDOW: type = "window"; break;
      case Type::REMOTE: type = "remote"; break;
      case Type::WEB_VIEW: type = "webview"; break;
      case Type::OFF_SCREEN: type = "offscreen"; break;
      default: break;
    }
    return mate::ConvertToV8(isolate, type);
  }
  static bool FromV8(v8::Isolate* isolate, v8::Local<v8::Value> val,