Harden Custom Element registration processing against context destruction.

This adds tests for destroying the containing iframe when:

- Retrieving the 'prototype' option during registration.
- Retrieving the 'ready' callback during registration.
- Upgrading an element as a result of registration.

It also adds a test for destroying the containing iframe, then trying
to register a new custom element.

BUG=234509
R=abarth@chromium.org

Review URL: https://codereview.chromium.org/18335002

git-svn-id: svn://svn.chromium.org/blink/trunk@153395 bbb929c8-8fbe-4397-9dbb-9b2b20218538