Fix parsing for quotes

Currently we accept strings like 'red' and '"a" "b" cake' when parsing
a quotes value. This patch makes these fail to parse. It also simplifies
the function parseQuotes to return a CSSValue, so we can use the code at
the bottom of the parseValue switch to handle trailing tokens and adding
the property.

BUG=404023

Review URL: https://codereview.chromium.org/498073002

git-svn-id: svn://svn.chromium.org/blink/trunk@180887 bbb929c8-8fbe-4397-9dbb-9b2b20218538