Reland "Explicitly wrap main thread in test_main.cc."

This is a reland of 711a31aead9007e42dd73c302c8ec40f9e931619

Changes since original landing:

Rename methods only used by tests, mainly via FakeClock,

  MessageQueueManager::ProcessAllMessageQueues
     --> ProcessAllMessageQueuesForTesting

  MessageQueue::IsProcessingMessages
     --> IsProcessingMessagesForTesting

Fix the handling of null rtc::Thread::Current() in
ProcessAllMessageQueuesInternal().

Add override Thread::IsProcessingMessagesForTesting() to return false
for the wrapped main thread, unless it's also the current thread. In
tests, the main thread is typically not processing any messages,
but blocked in an Event::Wait().

Original change's description:
> Explicitly wrap main thread in test_main.cc.
>
> Bug: webrtc:9714
> Change-Id: I6ee234f9a0b88b3656a683f2455c3e4b2acf0d54
> Reviewed-on: https://webrtc-review.googlesource.com/97683
> Reviewed-by: Tommi <tommi@webrtc.org>
> Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
> Commit-Queue: Niels Moller <nisse@webrtc.org>
> Cr-Commit-Position: refs/heads/master@{#24560}

Bug: webrtc:9714
Change-Id: I6f022d46aaf1e28f86f09f2d68c1803b69770126
Reviewed-on: https://webrtc-review.googlesource.com/98060
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Reviewed-by: Tommi <tommi@webrtc.org>
Commit-Queue: Niels Moller <nisse@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#24596}

rtc_base/thread.h

@@ -189,6 +189,7 @@ class RTC_LOCKABLE Thread : public MessageQueue {
   }
 
   // From MessageQueue
+  bool IsProcessingMessagesForTesting() override;
   void Clear(MessageHandler* phandler,
              uint32_t id = MQID_ANY,
              MessageList* removed = nullptr) override;