Clean up some misleading code in the apply_animation() macro

In both cases, we always loop from an index of zero, appending to the list as
required, so the list size can never be less than the index.

BUG=240653
R=timloh

Review URL: https://chromiumcodereview.appspot.com/23461034

git-svn-id: svn://svn.chromium.org/blink/trunk@157490 bbb929c8-8fbe-4397-9dbb-9b2b20218538