Implement default prototype handling for document.register.

When registering a Custom Element you have the right to have a
prototype present. If you do not provide a prototype, a
court-appointed prototype will be provided at no cost to represent
you.

BUG=234509

Review URL: https://chromiumcodereview.appspot.com/23558004

git-svn-id: svn://svn.chromium.org/blink/trunk@157027 bbb929c8-8fbe-4397-9dbb-9b2b20218538