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This was added at a time when there was interest in exploring more
animated transitions between icon states, but never had a client that
wasn't gated on a feature flag, and currently is dead code.

Bug: 718549
Change-Id: Idaf4667930e68feac92daafcab7c5778355546ef
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1986109
Commit-Queue: Evan Stade <estade@chromium.org>
Reviewed-by: Peter Kasting <pkasting@chromium.org>
Cr-Commit-Position: refs/heads/master@{#728111}