Web Animations CSS: Fix crash when animating viewport units

This patch makes viewport units default to snapping behaviour when
mismatched instead of animating them smoothly, this matches legacy
animation behaviour.

BUG=322675

Review URL: https://codereview.chromium.org/90113003

git-svn-id: svn://svn.chromium.org/blink/trunk@162956 bbb929c8-8fbe-4397-9dbb-9b2b20218538