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This shouldn't be necessary; when the last frame is deleted the observer
should remove itself. However, for paranoia's sake, remove the observers
as sessions are closed.

Bug: 1123557
Change-Id: If0109eafee7fd2ba38480849b99ee42bbafe5b61
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2424711
Commit-Queue: Leonard Grey <lgrey@chromium.org>
Auto-Submit: Avi Drissman <avi@chromium.org>
Reviewed-by: Leonard Grey <lgrey@chromium.org>
Cr-Commit-Position: refs/heads/master@{#809766}