This changes the work we do for document.open() in the following ways:
- We no longer create a new Window when doing document.open().
We use the same Window but remove all the event listeners on the
existing DOM tree and Window before removing the document's existing
children to provide a clean slate document to use for .write().
- We no longer create a session history entry (previously would be a
wyciwyg URI). We now replace the current one, effectively losing the
entry for the original document.
- We now support document.open() on windowless documents.
