Web Animations CSS: Don't start transitions for non-interpolable values

Transitions aren't supposed to start when a value change is between
non-interpolable values. This patch fixes this case so we don't
start transitions when we aren't supposed to.

BUG=303430

Review URL: https://codereview.chromium.org/28223003

git-svn-id: svn://svn.chromium.org/blink/trunk@160053 bbb929c8-8fbe-4397-9dbb-9b2b20218538