Cleanup SVG color parsing handling

Cleanup SVG color parsing handling, for stop-color, lighting-color and flood-color there is no need to test explicitly for
SVG extended colors, parseColor() can handle it. I was triggered by 'alpha' being placed within the SVG CSS extended color keywords, potentially leading to bugs, so move it out.

Review URL: https://codereview.chromium.org/392013002

git-svn-id: svn://svn.chromium.org/blink/trunk@178309 bbb929c8-8fbe-4397-9dbb-9b2b20218538