Allow Web Animations flags to be turned off as well as on

Allow Web Animations flags to be turned off as well as on

This is a first step towards switching to Web Animations as the default
implementation of CSS animations and transitions. When that switch is made, a
virtual test suite will be added to run the animations and transitions
LayoutTests with the legacy animations engine. Those virtual suites will
require a --disable-web-animations-css flag in Chromium, for which this change
is a prerequisite.

BUG=258896
R=abarth@chromium.org, dstockwell@chromium.org, dstockwell

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=161978

[dstockwell, 2013-11-11 12:01:04]

lgtm

[commit-bot: I haz the power, 2013-11-11 12:58:42]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/steveblock@chromium.org/66023004/1

[commit-bot: I haz the power, 2013-11-11 13:12:06]

Retried try job too often on blink_presubmit for step(s) presubmit
http://build.chromium.org/p/tryserver.chromium/buildstatus?builder=blink_pres...

[abarth-chromium, 2013-11-11 21:45:00]

lgtm

[commit-bot: I haz the power, 2013-11-11 21:45:12]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/steveblock@chromium.org/66023004/1

[Steve Block, 2013-11-14 04:29:01]

This has been waiting for the linux_blink bot for over 48 hours now, and is very
low risk. Landing manually.

[Steve Block, 2013-11-14 04:29:59]

Committed patchset #1 manually as r161978 (presubmit successful).