code: function broadcastReply(command, payload) {
      self.clients.matchAll()
          .then(clients => {
              for (const client of clients) {
                  client. /*OK*/ postMessage({
                      command,
                      payload,
                  });
              }
          });
  }
docs: Sends a postMessage() to all window frames the service worker controls.
  @param {string} command
  @param {!JsonObject} payload