Implement 'grid-template' parsing

This change makes us recognize grid-template, parse it, store it and
return properly to JavaScript.

The specification mandates to validate to check that the areas defined
by 'grid-template' are rectangular which involves a lot of the
CSSParser code.

Because validating involves building the grid areas, a new CSSValue
was introduced to hold the computed value. Note that we have to track
the explicit size of the named grid areas as the named grid areas (".")
are not tracked in our HashMap of grid areas.

BUG=258092
TEST=fast/css-grid-layout/grid-template-get-set.html

Review URL: https://chromiumcodereview.appspot.com/18532004

git-svn-id: svn://svn.chromium.org/blink/trunk@155199 bbb929c8-8fbe-4397-9dbb-9b2b20218538