Remove default values for Web Animations runtime flags

These are no longer required, as Chromium always sets an explict value.

BUG=258896
R=dstockwell

Review URL: https://codereview.chromium.org/73273002

git-svn-id: svn://svn.chromium.org/blink/trunk@162096 bbb929c8-8fbe-4397-9dbb-9b2b20218538