Add a test using Custom Elements from an isolated world.

For now, throw an exception if an extension tries to use
document.register.

BUG=234509

Review URL: https://chromiumcodereview.appspot.com/20698002

git-svn-id: svn://svn.chromium.org/blink/trunk@155003 bbb929c8-8fbe-4397-9dbb-9b2b20218538