[CSS Grid Layout] Fix grid-template serialization

This is a follow-up of https://codereview.chromium.org/18532004/#msg6

We didn't know if serializing strings was specified and it turns out
that it is: http://dev.w3.org/csswg/cssom/#serialize-a-string

Bottom-line, we should use double quote for
CSSGridTemplateValue serialization.

BUG=258092

Review URL: https://chromiumcodereview.appspot.com/22515002

git-svn-id: svn://svn.chromium.org/blink/trunk@155712 bbb929c8-8fbe-4397-9dbb-9b2b20218538