Create 2-keyframe transition animations with fill: backwards

A while ago we started creating 3-keyframe transition animations in
order to support a delay without having fill: both but it seems that
fill: backwards fixes the earlier bug and preserves the correct time for
events such as the transitionend event.

Bug: 658644
Change-Id: Iccbd65124209b3640e729833e05dfdb9d5685d96
Reviewed-on: https://chromium-review.googlesource.com/c/1440822
Reviewed-by: Stephen McGruer <smcgruer@chromium.org>
Commit-Queue: Robert Flack <flackr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#627471}
6 files changed