Remove the acceleratedCompositingForAnimationEnabled setting.

This is always on, so we don't need to branch on it.

Depends on: https://codereview.chromium.org/239973005/

R=abarth@chromium.org, vollick@chromium.org
BUG=366114

Review URL: https://codereview.chromium.org/249423002

git-svn-id: svn://svn.chromium.org/blink/trunk@172997 bbb929c8-8fbe-4397-9dbb-9b2b20218538