Merge SVGColor into SVGPaint

Merge SVGColor into SVGPaint and thus remove SVGColor. There is no need for this class anymore in SVG2. Instead we use CSSPrimitiveValue and add color handling to SVGPaint.

Review URL: https://codereview.chromium.org/189913012

git-svn-id: svn://svn.chromium.org/blink/trunk@168837 bbb929c8-8fbe-4397-9dbb-9b2b20218538