reinitializePromises_() {
    this.innerFrameEchoPayload_ = null;
    this.outerFrameEchoPayload_ = null;
    this.innerFrameResponseReceived_.promise.cancel();
    this.innerFrameResponseReceived_ = GoogPromise.withResolver();
    this.outerFrameResponseReceived_.promise.cancel();
    this.outerFrameResponseReceived_ = GoogPromise.withResolver();
  }
Initializes the promise resolvers and clears the echo payloads, ready for
another sub-test.
@private