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NavigationGlow can be destroyed after the window and view
associated with it, so we need to observe those events happening
and clean up accordingly.

OnDetachCompositor is called prior to the WindowAndroid being destroyed,
and surprisingly clears the observer list, so that is the best option
we have to clean up that reference.

BUG=1048851

Change-Id: I61e081dcab951ceeb6515a4e79e2d42ea2a96f84
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2065416
Commit-Queue: Ted Choc <tedchoc@chromium.org>
Reviewed-by: David Trainor <dtrainor@chromium.org>
Reviewed-by: Bo <boliu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#743316}