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It is common to animate something away via a transform, bounds, or
opacity animation and want to flip a visibility bit upon completion of
that transition. Boolean animations are suited to this task. They act
as a step function returning the second value when the timing function
returns a fraction of 1.0 (and you can use this timing function to
adjust the position of the step).

This change also finishes the small bit of plumbing to tick bg
color animations.

Bug: None
Cq-Include-Trybots: master.tryserver.blink:linux_trusty_blink_rel
Change-Id: I18ab39570e2de63cbf103a36c10ea1766efc12e8
Reviewed-on: https://chromium-review.googlesource.com/579529
Commit-Queue: Ian Vollick <vollick@chromium.org>
Reviewed-by: Ali Juma <ajuma@chromium.org>
Cr-Commit-Position: refs/heads/master@{#488311}