Revert of Improving the fake clock and using it to fix a flaky STUN timeout test. (patchset #10 id:180001 of https://codereview.webrtc.org/2024813004/ )

Reason for revert:
There seems to be a TSan warning that wasn't caught by the trybot: https://build.chromium.org/p/client.webrtc/builders/Linux%20Tsan%20v2/builds/6732/steps/peerconnection_unittests/logs/stdio

Apparently a thread is still alive even after destroying WebRTCSession. Need to think of a way to fix this, without adding a critical section around g_clock (since that would hurt performance).

Original issue's description:
> Improving the fake clock and using it to fix a flaky STUN timeout test.
>
> When the fake clock's time is advanced, it now ensures all pending
> queued messages have been dispatched. This allows us to write a
> "SIMULATED_WAIT" macro that ticks the simulated clock by milliseconds up
> until the target time.
>
> Useful in this case, where we know the STUN timeout should take a total
> of 9500ms, but it would be overly complex to write test code that waits
> for each individual timeout, ensures a STUN packet has been
> retransmited, etc.
>
> (The test described above *should* be written, but it belongs in
> p2ptransportchannel_unittest.cc, not webrtcsession_unittest.cc).
>
> Committed: https://crrev.com/ffbe0e17e2c9b7fe101023acf40574dc0c95631a
> Cr-Commit-Position: refs/heads/master@{#13043}

TBR=pthatcher@webrtc.org,tommi@webrtc.org
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true

Review-Url: https://codereview.webrtc.org/2038213002
Cr-Commit-Position: refs/heads/master@{#13045}
diff --git a/webrtc/base/messagequeue.h b/webrtc/base/messagequeue.h
index bf11037..3a5226c 100644
--- a/webrtc/base/messagequeue.h
+++ b/webrtc/base/messagequeue.h
@@ -37,7 +37,7 @@
 
 // MessageQueueManager does cleanup of of message queues
 
-class MessageQueueManager {
+class MessageQueueManager : public MessageHandler {
  public:
   static void Add(MessageQueue *message_queue);
   static void Remove(MessageQueue *message_queue);
@@ -50,20 +50,21 @@
   static bool IsInitialized();
 
   // Mainly for testing purposes, for use with a simulated clock.
-  // Ensures that all message queues have processed delayed messages
-  // up until the current point in time.
-  static void ProcessAllMessageQueues();
+  // Posts a no-op event on all message queues so they will wake from the
+  // socket server select() and process messages again.
+  static void WakeAllMessageQueues();
 
  private:
   static MessageQueueManager* Instance();
 
   MessageQueueManager();
-  ~MessageQueueManager();
+  ~MessageQueueManager() override;
 
   void AddInternal(MessageQueue *message_queue);
   void RemoveInternal(MessageQueue *message_queue);
   void ClearInternal(MessageHandler *handler);
-  void ProcessAllMessageQueuesInternal();
+  void WakeAllMessageQueuesInternal();
+  void OnMessage(Message* pmsg) override;
 
   static MessageQueueManager* instance_;
   // This list contains all live MessageQueues.