Avoid mishandling CSSPendingSubstitutionValues in CSS animations

This change ensures we have a code path for CSSPendingSubstitutionValues
during animation, it does not attempt to perform the correct animated
behaviour just yet.

BUG=627756

Review-Url: https://codereview.chromium.org/2146053003
Cr-Commit-Position: refs/heads/master@{#405699}
2 files changed