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

            callback(content);
        }
@param {number} statusCode
@param {!Object.<string, string>} headers
@param {string} content
@this {WebInspector.CompilerSourceMappingContentProvider}