Use consumeComma instead of manually checking for a comma operator

Using the helper function consumeComma is a bit nicer than explicitly
checking for commas all over the place, especially as it will allow us
to more easily change the interface of CSSParserValueList if we want to.

BUG=404023

Review URL: https://codereview.chromium.org/500913002

git-svn-id: svn://svn.chromium.org/blink/trunk@180885 bbb929c8-8fbe-4397-9dbb-9b2b20218538