Web Animations CSS: Support Animation of shape-inside and clip-path

Since this defers to the existing interpolation logic some errors around
clamping of radii are preserved.

Note that shape-inside is not yet shipped.

BUG=257591, 302068

Review URL: https://codereview.chromium.org/25191002

git-svn-id: svn://svn.chromium.org/blink/trunk@158613 bbb929c8-8fbe-4397-9dbb-9b2b20218538