Web Animations CSS: Re-enable Web Animation perf benchmark

The blink_perf.web_animations benchmark was disabled in
https://codereview.chromium.org/48793006.
These Web Animation perf tests no longer crash and can be re-enabled to
be run by the perf bots.

NOTRY=TRUE
BUG=309981

Review URL: https://codereview.chromium.org/55363006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@232910 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 232910