Web Animations CSS: Support animation of font-size

BUG=257591

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=159448

Review URL: https://codereview.chromium.org/26320008

git-svn-id: svn://svn.chromium.org/blink/trunk@159549 bbb929c8-8fbe-4397-9dbb-9b2b20218538