Web Animations: Implement AnimatableTransform

This patch adds AnimatableTransform for animating transform lists. This
also adds TransformOperations::add (this could go in AnimatableTransform
alternatively, but I think it's cleaner to put the logic here) for the
add operation.

BUG=257591

Review URL: https://chromiumcodereview.appspot.com/22371011

git-svn-id: svn://svn.chromium.org/blink/trunk@155984 bbb929c8-8fbe-4397-9dbb-9b2b20218538