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This will make using AnyWidgetWaiter a lot more convenient for tests
that cover code which does not subclass View for the widget delegate's
contents view (which is current best practice).

Bug: 1075649
Change-Id: Ie1c8d2917f525176d1b17bcbeddd7b6f93b19644
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2492891
Commit-Queue: Elly Fong-Jones <ellyjones@chromium.org>
Reviewed-by: Peter Boström <pbos@chromium.org>
Cr-Commit-Position: refs/heads/master@{#821285}