Merge SVGCSSParser into CSSPropertyParser.cpp

Because we always compile SVG there is no need anymore to have this in a separate file. Also now we end up with only one isSystemColor.

Review URL: https://codereview.chromium.org/184693003

git-svn-id: svn://svn.chromium.org/blink/trunk@168179 bbb929c8-8fbe-4397-9dbb-9b2b20218538