Handle "url(...) none" and "url(...) currentColor" SVG paint specs

Consider all paint types with a URI in paintingResourceFromSVGPaint -
URI_NONE, URI_CURRENTCOLOR and URI_RGBCOLOR_ICCCOLOR were previously
not considered. (The last one, URI_RGBCOLOR_ICCCOLOR, will never appear
out of the parser.)
Add parsing support for the URI_CURRENTCOLOR type, since it's trivial.

TEST=svg/custom/fill-fallback-{none,currentcolor-{1,2}}.svg
BUG=162547

Review URL: https://codereview.chromium.org/108303009

git-svn-id: svn://svn.chromium.org/blink/trunk@164200 bbb929c8-8fbe-4397-9dbb-9b2b20218538