Fix animation of points attribute for polygons

This is a merge of http://trac.webkit.org/changeset/159559 by Antoine
Quint <graouts@apple.com>.

This patch switches to using the animated points value instead of the
non-animated points value in SVGPathData, fixing a bug where points
could not be animated.

BUG=321710

Review URL: https://codereview.chromium.org/79063004

git-svn-id: svn://svn.chromium.org/blink/trunk@162399 bbb929c8-8fbe-4397-9dbb-9b2b20218538