[CSS Shapes] add shape-margin support for image valued shapes

Add support for computing the shape-margin boundary for a shape-outside whose value is an image.

BUG=299813

Review URL: https://codereview.chromium.org/25008004

git-svn-id: svn://svn.chromium.org/blink/trunk@158558 bbb929c8-8fbe-4397-9dbb-9b2b20218538