Web Animations: Simplify mutable AnimationTimeline currentTime and playbackRate

Web Animations: Simplify mutable AnimationTimeline currentTime and playbackRate

This uses the same model as AnimationPlayer, treating zeroTime as the
startTime of the timeline. This resolves some issues where the setter
did not set a consistent value (see test) and ensures that with
playbackRate 0, the currentTime is identical to the set value.

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=188428

[dstockwell, 2015-01-14 12:50:40]

dstockwell@chromium.org changed reviewers:
+ samli@chromium.org

[dstockwell, 2015-01-14 12:50:41]

Ran into some issues and decided to overhaul this. I think this was complex
previously while we were trying to support compositor animations. I think we'll
have better success in the long term through communicating changes to the
playback rate and zero time with the compositor.

[dstockwell, 2015-01-14 12:55:40]

https://codereview.chromium.org/847283004/diff/20001/LayoutTests/web-animatio...
File LayoutTests/web-animations-api/animationtimeline-consistent.html (right):

https://codereview.chromium.org/847283004/diff/20001/LayoutTests/web-animatio...
LayoutTests/web-animations-api/animationtimeline-consistent.html:14:
assert_approx_equals(document.timeline.currentTime, 100, 0.1);
this was coming out ~75 for me before this change

[samli, 2015-01-14 23:55:50]

lgtm

[dstockwell, 2015-01-15 00:05:24]

The CQ bit was checked by dstockwell@chromium.org

[commit-bot: I haz the power, 2015-01-15 00:06:05]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/847283004/40001

[commit-bot: I haz the power, 2015-01-15 01:31:40]

Committed patchset #3 (id:40001) as
https://src.chromium.org/viewvc/blink?view=rev&revision=188428