function textCallback(error, text)
        {
            if (error || text === null) {
                WebInspector.console.error("Failed to get text for stylesheet " + this.id + ": " + error)
                text = "";
                // Fall through.
            }
            return this._trimSourceURL(text);
        }
@param {?Protocol.Error} error
@param {?string} text
@return {string}
@this {WebInspector.CSSStyleSheetHeader}