function contentLoaded(error, statusCode, headers, content)
        {
            if (error || statusCode >= 400) {
                console.error("Could not load content for " + this._sourceURL + " : " + (error || ("HTTP status code: " + statusCode)));
                callback(null);
                return;
            }

            callback(content);
        }
@param {?Protocol.Error} error
@param {number} statusCode
@param {!NetworkAgent.Headers} headers
@param {string} content
@this {WebInspector.CompilerSourceMappingContentProvider}