Disallow an odd number of strings when parsing the quotes property

The spec only allows an even number of strings in the quotes property,
but currently we will accept an odd number of strings as a valid value.

http://www.w3.org/TR/CSS2/generate.html#quotes

BUG=389724

Review URL: https://codereview.chromium.org/360583004

git-svn-id: svn://svn.chromium.org/blink/trunk@177195 bbb929c8-8fbe-4397-9dbb-9b2b20218538