Remove SVG paint order runtime flag

Remove SVG paint order runtime flag, since this has been stable for a while. This also removes one check in CSSPropertyParser.

Review URL: https://codereview.chromium.org/339963002

git-svn-id: svn://svn.chromium.org/blink/trunk@176372 bbb929c8-8fbe-4397-9dbb-9b2b20218538