[CSS Grid Layout] Update named grid lines syntax to the last version of the specs

Names for grid lines are now specified as a list of whitespace separated
idents enclosed in parentheses instead of as a list of strings.

BUG=282452

Review URL: https://codereview.chromium.org/23528004

git-svn-id: svn://svn.chromium.org/blink/trunk@162555 bbb929c8-8fbe-4397-9dbb-9b2b20218538