Web Animations CSS: Handle transitions on unanimatable properties nicely

This patch stops us hitting an ASSERT (on creating the AnimableValue)
when we have a transition on an unanimatable property.
Note that we can't filter these out earlier as the spec[1] says:
 "unrecognized or non-animatable properties must be kept in the list"

[1] http://www.w3.org/TR/css3-transitions/#transition-property-property

BUG=303430

Review URL: https://codereview.chromium.org/39553002

git-svn-id: svn://svn.chromium.org/blink/trunk@160888 bbb929c8-8fbe-4397-9dbb-9b2b20218538