docs: function ensureImageLoaded(url) {
              var img = new Image();
              var destroy = function () {
                  if (img) {
                      img.src = ""; // 読み込み停止
                      img = null;
                  }
              };
              return new Promise(function (resolve, reject) {
                  img.onload = function (event) {
                      destroy();
                      resolve(url);
                  };
                  img.onerror = function (event) {
                      destroy();
                      reject(CDP.makeErrorInfo(CDP.RESULT_CODE.ERROR_CDP_TOOLS_IMAGE_LOAD_FAILED, TAG, "image load failed. [url: " + url + "]"));
                  };
                  img.src = url;
              }, destroy);
          }