Web Animations: Retire StringKeyframe setPropertyCSSValue declaration

This method was declared but never defined or called.

R=alancutter

Review URL: https://codereview.chromium.org/240003007

git-svn-id: svn://svn.chromium.org/blink/trunk@171842 bbb929c8-8fbe-4397-9dbb-9b2b20218538