Make sure that "current" rtc::Thread instances are always current for TaskQueueBase.

This is a necessary part of fulfilling the TaskQueueBase
interface. If a thread does not register as the current TQ, yet offers
the TQ interface, TQ 'current' checks will not work as expected and
code that relies them (TaskQueueBase::Current() and IsCurrent())
will run in unexpected ways.

Bug: webrtc:11572
Change-Id: Iab747bc474e74e6ce4f9e914cfd5b0578b19d19c
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/175080
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Commit-Queue: Tommi <tommi@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#31254}

rtc_base/thread.h

@@ -551,6 +551,12 @@ class RTC_LOCKABLE RTC_EXPORT Thread : public webrtc::TaskQueueBase {
   void InvokeInternal(const Location& posted_from,
                       rtc::FunctionView<void()> functor);
 
+  // Called by the ThreadManager when being set as the current thread.
+  void EnsureIsCurrentTaskQueue();
+
+  // Called by the ThreadManager when being unset as the current thread.
+  void ClearCurrentTaskQueue();
+
   // Returns a static-lifetime MessageHandler which runs message with
   // MessageLikeTask payload data.
   static MessageHandler* GetPostTaskMessageHandler();
@@ -595,6 +601,8 @@ class RTC_LOCKABLE RTC_EXPORT Thread : public webrtc::TaskQueueBase {
 
   // Runs webrtc::QueuedTask posted to the Thread.
   QueuedTaskHandler queued_task_handler_;
+  std::unique_ptr<TaskQueueBase::CurrentTaskQueueSetter>
+      task_queue_registration_;
 
   friend class ThreadManager;