getRawDataSize() {
    'use strict';
    if (goog.labs.net.webChannel.Wire.RAW_DATA_KEY in this.map) {
      const data = this.map[goog.labs.net.webChannel.Wire.RAW_DATA_KEY];
      if (typeof data === 'string') {
        return data.length;
      }
    }

    return undefined;
  }
@return {number|undefined} the size of the raw JSON message or
undefined if the message is not encoded as a raw JSON message