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
PrintSettings* out) {
    mate::Dictionary dict;
    if (!ConvertFromV8(isolate, val, &dict))
      return false;
    dict.Get("silent", &(out->silent));
    dict.Get("printBackground", &(out->print_background));
    return true;
  }
};
template<>
struct Converter<WindowOpenDisposition> {
  static v8::Local<v8::Value> ToV8(v8::Isolate* isolate,
                                   WindowOpenDisposition val) {
    std::string disposition = "other";
    switch (val) {
      case CURRENT_TAB: disposition = "default"; break;
      case NEW_FOREGROUND_TAB: disposition = "foreground-tab"; break;
      case NEW_BACKGROUND_TAB: disposition = "background-tab"; break;
      case NEW_POPUP: case NEW_WINDOW: disposition = "new-window"; break;
      default: break;
    }
    return mate::ConvertToV8(isolate, disposition);
  }
};
template<>
struct Converter<net::HttpResponseHeaders*> {
  static v8::Local<v8::Value> ToV8(v8::Isolate* isolate,