[CSS Grid Layout] Improve verbosity of test results

Recent refactorings in the grid layout testing code have worsen the verbosity
of our results. This cl brings back the use of the element ID for the test
expectations.

Review URL: https://codereview.chromium.org/59473002

git-svn-id: svn://svn.chromium.org/blink/trunk@161363 bbb929c8-8fbe-4397-9dbb-9b2b20218538