Add some layout tests to explicitly test dynamic translateZ changes.

R=enne@chromium.org
BUG=361729

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=171191

Review URL: https://codereview.chromium.org/231843002

git-svn-id: svn://svn.chromium.org/blink/trunk@171286 bbb929c8-8fbe-4397-9dbb-9b2b20218538