Enable animation for the shape-outside property

Add shape-outside to the list of animatable CSS properties, using the
same interpolator as shape-inside.

BUG=304916

Review URL: https://codereview.chromium.org/26546007

git-svn-id: svn://svn.chromium.org/blink/trunk@159730 bbb929c8-8fbe-4397-9dbb-9b2b20218538