Enable animation for the shape-image-threshold property

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

BUG=304916

Review URL: https://codereview.chromium.org/26988004

git-svn-id: svn://svn.chromium.org/blink/trunk@162643 bbb929c8-8fbe-4397-9dbb-9b2b20218538