'grid-template' shouldn't be exposed if grid is disabled

When implementing the property, it wasn't added to the CSS
Grid Layout properties' list and thus was wrongly exposed
if the runtime flag was disabled.

Review URL: https://chromiumcodereview.appspot.com/22876026

git-svn-id: svn://svn.chromium.org/blink/trunk@156601 bbb929c8-8fbe-4397-9dbb-9b2b20218538