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These instances were found by adding a CHECK to the WidgetObserver DTOR
to ensure the observer is removed before destruction and then running
all tests.

Bug: 1055483
Change-Id: I83353f33962b00f0f922a21ebb0afb2131295482
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2165105
Commit-Queue: Richard Knoll <knollr@chromium.org>
Reviewed-by: Toni Baržić <tbarzic@chromium.org>
Reviewed-by: Peter Kasting <pkasting@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#763422}