SVG2: Ship 'paint-order' property.

Intent-to-ship thread: https://groups.google.com/a/chromium.org/forum/#!topic/blink-dev/ccVN9-oSwe8

BUG=343496

Review URL: https://codereview.chromium.org/181233003

git-svn-id: svn://svn.chromium.org/blink/trunk@167915 bbb929c8-8fbe-4397-9dbb-9b2b20218538