[CSS Grid Layout] Rename named areas property

The property has been renamed from grid-template to grid-template-areas
in the last two versions of the spec.

BUG=337626

Review URL: https://codereview.chromium.org/145013017

git-svn-id: svn://svn.chromium.org/blink/trunk@165891 bbb929c8-8fbe-4397-9dbb-9b2b20218538