Simplify fallback-handling for SVG_PAINTTYPE_URI_*

De-duplicate some of the logic for determining a fallback for a
non-existant url(...). This also fixes the case where a SVGResources
object is available, but the fallback is "none".

BUG=415622

Review URL: https://codereview.chromium.org/577283003

git-svn-id: svn://svn.chromium.org/blink/trunk@182253 bbb929c8-8fbe-4397-9dbb-9b2b20218538