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This method was a relic from an earlier version of this code, when the
animation faded the path from a given starting color to a given ending
color. In that version, HasStarted() was necessary to avoid duplicate
animations, which would cause a flicker. For example, an initial user
interaction would start to animate the path from opaque to transparent,
and a subsequent user interaction while still animating could restart
the same animation, causing the path to flicker from a midpoint color
back to opaque to start the animation again. To avoid this flicker, I
introduced HasStarted() to avoid running a given animation more than
once.

But the new ElideAnimation version of the code animates from whatever
the current state is to the desired end state. This means that
restarting a duplicate animation is a no-op and doesn't cause any
flicker, so we can get rid of HasStarted(). I've also added unit tests
for the cases where HasStarted() was used.

Bug: 1099875
Change-Id: I53af1ae2437fbe24e6646c99d743f0af2a398ffa
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2278164
Commit-Queue: Emily Stark <estark@chromium.org>
Reviewed-by: Justin Donnelly <jdonnelly@chromium.org>
Cr-Commit-Position: refs/heads/master@{#785747}