Remove SVGPaint

Remove SVGPaint usage, instead use CSSValueList and CSSPrimitiveValue
for the fill/stroke CSS values.
This allows us to remove SVGPaint specific code in CSSValue.

Note that new result for svg/css/svg-attribute-parser-mode.svg matches
FireFox.

Review URL: https://codereview.chromium.org/361543002

git-svn-id: svn://svn.chromium.org/blink/trunk@177550 bbb929c8-8fbe-4397-9dbb-9b2b20218538