Add a consumeComma function to make some parsing routines nicer

Currently a lot of property parsing routines involve checking isComma
and then calling next on the CSSParserValueList. Having a consumeComma
function here makes these a bit nicer.

BUG=404023

Review URL: https://codereview.chromium.org/495093004

git-svn-id: svn://svn.chromium.org/blink/trunk@180874 bbb929c8-8fbe-4397-9dbb-9b2b20218538