Use count strings as keyframes/animation-name identifiers

Strings are not valid keyframes/animation-name identifiers. This change
tracks the occurrence of these strings in the wild to provide data
on the impact dropping support for this would have.

This is a reland of https://codereview.chromium.org/651123003 with
more null checks.

BUG=423424

Review URL: https://codereview.chromium.org/668613002

git-svn-id: svn://svn.chromium.org/blink/trunk@184033 bbb929c8-8fbe-4397-9dbb-9b2b20218538