GitLab

Replace explicit thread usage in ObserverListThreadSafeTest with a
number of tasks that run on dedicated worker pool threads.

We also replace an instance of MessageLoop in WeakNsObjectTest with a
scoped task environment.

Bug: 891670
Change-Id: I3b65f9b3ed54a42f40f38e98ef9b890d21003368
Reviewed-on: https://chromium-review.googlesource.com/c/1350914
Commit-Queue: Sami Kyöstilä <skyostil@chromium.org>
Reviewed-by: François Doray <fdoray@chromium.org>
Cr-Commit-Position: refs/heads/master@{#612141}