Change SVG paint server fallback for a non-existent URL

SVG2 changed the behavior from "...the document is in error" to
"...it must be treated as if none was specified".

BUG=416580

Review URL: https://codereview.chromium.org/593173002

git-svn-id: svn://svn.chromium.org/blink/trunk@182616 bbb929c8-8fbe-4397-9dbb-9b2b20218538