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

tools/perf/benchmarks/blink_perf.py

@@ -31,7 +31,6 @@ class BlinkPerfAnimation(test.Test):
 class BlinkPerfWebAnimations(test.Test):
   tag = 'web_animations'
   test = blink_perf.BlinkPerfMeasurement
-  enabled = False
 
   def CreatePageSet(self, options):
     path = os.path.join(util.GetChromiumSrcDir(),