Revert of Web Animations: Use a single animation clock

Revert of Web Animations: Use a single animation clock (patchset #6 id:100001 of https://codereview.chromium.org/1410313004/ )

Reason for revert:
Caused animation tests to be flaky on all platforms, especially Android; see http://crbug.com/549318 .

Original issue's description:
> Web Animations: Use a single animation clock
>
> All animation clocks for a page should tick at the same time, but since we
> had an animation clock per document there were some cases where clocks
> could tick independently.
>
> This patch moves the clock from Document to PageAnimator.
> animation-timeline-detached-no-crash.html is removed as the test was not valid
> due to the animation's document not being active.
>
> Committed: https://crrev.com/b45c39903df3727d21cbc217a0b14c2c85b0715a
> Cr-Commit-Position: refs/heads/master@{#356776}

TBR=samli@chromium.org,dstockwell@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true

Committed: https://crrev.com/7cd7ee8af210ae1e11328ea449e2e54befb059b0
Cr-Commit-Position: refs/heads/master@{#356960}

[Peter Kasting, 2015-10-29 22:02:08]

Created Revert of Web Animations: Use a single animation clock

[commit-bot: I haz the power, 2015-10-29 22:05:37]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1417613005/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1417613005/1

[commit-bot: I haz the power, 2015-10-29 22:07:38]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-10-29 22:08:32]

Patchset 1 (id:??) landed as
https://crrev.com/7cd7ee8af210ae1e11328ea449e2e54befb059b0
Cr-Commit-Position: refs/heads/master@{#356960}