Remove the WebAnimationsCSS runtime feature flag

Feature was promoted to stable in r162935[1]
Legacy animations engine was removed in r166881[2]

[1] http://src.chromium.org/viewvc/blink?view=rev&rev=162935
[2] http://src.chromium.org/viewvc/blink?view=rev&rev=166881

BUG=334225

Review URL: https://codereview.chromium.org/155763006

git-svn-id: svn://svn.chromium.org/blink/trunk@167058 bbb929c8-8fbe-4397-9dbb-9b2b20218538