[CSS Grid Layout] Add element.style coverage for grid-auto-flow

As follow-up patch for r177258 we're adding new test cases for
grid-auto-flow to check the correct behavior of element.style.

Review URL: https://codereview.chromium.org/360233002

git-svn-id: svn://svn.chromium.org/blink/trunk@178187 bbb929c8-8fbe-4397-9dbb-9b2b20218538