GitLab

If all the sequences has been detached, the observer should be notified,
otherwise it might wait forever.

Also added unit test cases for this scenario.

Bug: b/141997678
Test: Run CallbackLayerAnimationObserverTest.* in compositor_unittests
Change-Id: I3a025de8e635a383910ef4a3e650683478e8d3fc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1938459Reviewed-by: Robert Flack <flackr@chromium.org>
Reviewed-by: Xiaohui Chen <xiaohuic@chromium.org>
Commit-Queue: Yue Li <updowndota@chromium.org>
Cr-Commit-Position: refs/heads/master@{#721580}