Add a test that Custom Elements do not induce leaks.

BUG=234509
R=abarth@chromium.org

Review URL: https://codereview.chromium.org/21060003

git-svn-id: svn://svn.chromium.org/blink/trunk@155189 bbb929c8-8fbe-4397-9dbb-9b2b20218538