GitLab

Use ObserverListThreadSafe for CameraActiveClientObserver instead of
ObserverList. Also add a lock to guard the list.

Bug: b/170075468
Test: simplechrome
Change-Id: I976200eed4e24f036e96481a145250aa90fdc3fb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2566329Reviewed-by: Wei Lee <wtlee@chromium.org>
Commit-Queue: Jasmine Chen <lnishan@google.com>
Cr-Commit-Position: refs/heads/master@{#833613}