Remove document param from parseSystemColor

The document parameter was introduced in http://trac.webkit.org/changeset/78442 because it was needed to get to the system colors. However this now can be accessed using a RenderTheme singleton so remove the parameter.

Review URL: https://codereview.chromium.org/196093002

git-svn-id: svn://svn.chromium.org/blink/trunk@169063 bbb929c8-8fbe-4397-9dbb-9b2b20218538