Move CSSParserString out of CSSParser.cpp

There were several CSSParserString functions that were wrongly
defined into CSSParser.cpp. They are moved to CSSParserValues.*.

Review URL: https://codereview.chromium.org/73753002

git-svn-id: svn://svn.chromium.org/blink/trunk@162093 bbb929c8-8fbe-4397-9dbb-9b2b20218538