Remove the PendingTaskSafetyFlag::Pointer type add ScopedTaskSafety.

ScopedTaskSafety simplifies usage of PendingTaskSafetyFlag,
so this CL also includes ToQueuedTask support for ScopedTaskSafety
and test updates.

This is following up on feedback in the following CL:
https://webrtc-review.googlesource.com/c/src/+/174262

Change-Id: Idd38dfc1914b24a05fdc4ad256b409dcf1795fc0
Bug: none
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/174740
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Commit-Queue: Tommi <tommi@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#31241}

video/rtp_streams_synchronizer2.h

@@ -70,8 +70,7 @@ class RtpStreamsSynchronizer {
   bool timer_running_ RTC_GUARDED_BY(main_checker_) = false;
 
   // Used to signal destruction to potentially pending tasks.
-  PendingTaskSafetyFlag::Pointer task_safety_flag_ =
-      PendingTaskSafetyFlag::Create();
+  ScopedTaskSafety task_safety_;
 };
 
 }  // namespace internal