Add a RemoveObserverPolicy parameter to ObserverListThreadSafe

The policy defaults to kAnySequence, the existing behaviour, but can be
set to kAddingSequenceOnly to CHECK that observers are only removed from
the same sequence that added them. This prevents a mistake where an
observer is deleted immediately after calling RemoveObserver from a
different sequence, even though it might still be inside a callback.

Also adds 3 more cross-thread ObserverListThreadSafeTest's, and reduces
the duration of each from 2s to 1s to compensate.

R=fdoray

Bug: 1471683
Change-Id: I73e9cc21f2186ebdcf8f2dacd1a71aa4d11134c7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4781112


Commit-Queue: Matt Mueller <mattm@chromium.org>
Reviewed-by: Matt Mueller <mattm@chromium.org>
Reviewed-by: Francois Pierre Doray <fdoray@chromium.org>
Auto-Submit: Joe Mason <joenotcharles@google.com>
Cr-Commit-Position: refs/heads/main@{#1184841}