Web Animations CSS: Use legacy equality function to determine whether to start transitions

Constructing AnimatableValues for the purpose of deciding whether a transition should start was causing a mild performance regression when large numbers of properties were targeted for transition.

This patch reverts to the old-style comparison function from frame/animation, and fixes that regression.

BUG=303430

Review URL: https://codereview.chromium.org/54543006

git-svn-id: svn://svn.chromium.org/blink/trunk@161239 bbb929c8-8fbe-4397-9dbb-9b2b20218538