Web Animations: Don't set compositor pending when document has been detached

Web Animations: Don't set compositor pending when document has been detached

When there are multiple documents it is possible for an animation's
timeline to become detached. The existing assertions were not valid in this
case.

BUG=492061
Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=202666

[dstockwell, 2015-09-23 00:56:03]

The CQ bit was checked by dstockwell@chromium.org to run a CQ dry run

[dstockwell, 2015-09-23 00:56:10]

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

[commit-bot: I haz the power, 2015-09-23 00:56:33]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1360043002/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1360043002/1

[alancutter (OOO until 2018), 2015-09-23 03:57:57]

> When there were multiple documents it was possible for an animation's
timeline to become detached.

Should this be in present tense?

[alancutter (OOO until 2018), 2015-09-23 03:59:22]

lgtm

[commit-bot: I haz the power, 2015-09-23 04:57:05]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2015-09-23 04:57:06]

Dry run: Try jobs failed on following builders:
  linux_android_rel_ng on tryserver.chromium.linux (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.linux/builders/linux_android_r...)

[dstockwell, 2015-09-23 05:19:42]

The CQ bit was checked by dstockwell@chromium.org

[commit-bot: I haz the power, 2015-09-23 05:20:07]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1360043002/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1360043002/1

[commit-bot: I haz the power, 2015-09-23 06:33:43]

Committed patchset #1 (id:1) as
https://src.chromium.org/viewvc/blink?view=rev&revision=202666