message: Add WaitForPreviouslyPostedTasks to TaskQueueForTest.
  Add an utility function to TaskQueueForTest to execute all already
  posted tasks on the queue.
  Cr-Commit-Position: refs/heads/master@{#31152}
diff: @@ -66,6 +66,14 @@ class RTC_LOCKABLE TaskQueueForTest : public rtc::TaskQueue {
  void SendTask(Closure&& task, rtc::Location loc) {
  ::webrtc::SendTask(loc, Get(), std::forward<Closure>(task));
  }
  +
  + // Wait for the completion of all tasks posted prior to the
  + // WaitForPreviouslyPostedTasks() call.
  + void WaitForPreviouslyPostedTasks() {
  + // Post an empty task on the queue and wait for it to finish, to ensure
  + // that all already posted tasks on the queue get executed.
  + SendTask([]() {}, RTC_FROM_HERE);
  + }
  };
  } // namespace webrtc
  