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This lets us avoid some ViewHierarchyChanged() overrides.

BUG=none
TEST=none

Change-Id: I7896460f65cedf2351f5acd3e16893674faa72ea
Reviewed-on: https://chromium-review.googlesource.com/996908
Commit-Queue: Peter Kasting <pkasting@chromium.org>
Reviewed-by: Evan Stade <estade@chromium.org>
Cr-Commit-Position: refs/heads/master@{#548846}