Add a consumeComma function to make some parsing routines nicer
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: bf5cd6ccde378db821296732a091cfbcf5285fbd@180874 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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