Rewrite animationtimeline-consistent into animation-playbackRate

This tests that the animation's currentTime is constant when its
playbackRate is 0.

This test is intended for upstreaming in
interfaces/Animation/playbackRate.html

BUG=623437

Review-Url: https://codereview.chromium.org/2137403002
Cr-Commit-Position: refs/heads/master@{#404795}
2 files changed