[CSS Grid Layout] Add layout test for anonymous grid items

Add a layout test to prevent regressions supporting anonymous grid
items.

TEST=fast/css-grid-layout/anonymous-grid-items.html

Review URL: https://codereview.chromium.org/181183005

git-svn-id: svn://svn.chromium.org/blink/trunk@168680 bbb929c8-8fbe-4397-9dbb-9b2b20218538