Add ids to make debugging easier.

These don't add anything to the test, but if you're using trace-to-console it
makes it a lot easier to identify which repaint call goes with which div.

Review URL: https://codereview.chromium.org/267283006

git-svn-id: svn://svn.chromium.org/blink/trunk@173947 bbb929c8-8fbe-4397-9dbb-9b2b20218538