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}