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An edge case around direction and playback rate is calculating the time
incorrectly. In particular, when the playback rate is negative and the
direction is set to AlternateReverse, the resulting iteration times
calculated are incorrect. This change brings the compositor in line
with the Web Animation Spec to calculate the iteration time.

Reference for calculations:
http://dev.w3.org/fxtf/web-animations/#core-animation-node-calculations

BUG=398330

Review URL: https://codereview.chromium.org/565353003

Cr-Commit-Position: refs/heads/master@{#294708}