[CSS Grid Layout] Add support for named grid areas

This is the final change in adding support for named grid areas:
extending the resolution code to handle named grid areas.

The patch is trivial as it builds on a lot of the
existing grid handling code.

BUG=258092

Review URL: https://chromiumcodereview.appspot.com/22759002

git-svn-id: svn://svn.chromium.org/blink/trunk@155889 bbb929c8-8fbe-4397-9dbb-9b2b20218538