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.

BUG=423424

Review URL: https://codereview.chromium.org/651123003

git-svn-id: svn://svn.chromium.org/blink/trunk@183856 bbb929c8-8fbe-4397-9dbb-9b2b20218538