Revert "Web Animations CSS: Support animation of font-size"

This reverts r159448. It caused virtual/web-animations-css/animations/interpolation/flex-interpolation.html
to hit an assert.

TBR=ericwilligers@chromium.org

Review URL: https://codereview.chromium.org/26817005

git-svn-id: svn://svn.chromium.org/blink/trunk@159460 bbb929c8-8fbe-4397-9dbb-9b2b20218538