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Widget observers need to make sure they unregister themselves before
they are destroyed.

Bug: 1055483
Change-Id: I53c78555a9cd9d28fe7edab0630bca17f90e47a7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2137319Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
Reviewed-by: Peter Beverloo <peter@chromium.org>
Reviewed-by: Peter Kasting <pkasting@chromium.org>
Commit-Queue: Richard Knoll <knollr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#757412}