Enable animation for the shape-margin property

Changes to the shape-margin should relayout any content affected by the shape's
contour. This patch makes shape-margin respond to dynamic changes made via JS
and CSS animations.

BUG=304916

Review URL: https://codereview.chromium.org/26427004

git-svn-id: svn://svn.chromium.org/blink/trunk@160161 bbb929c8-8fbe-4397-9dbb-9b2b20218538