Based on past discussions I'm going to try to reland this change. This makes window.document and document behave the same after navigation, which is a change from what the spec says. If this works out though, it would greatly simplify the spec; and fix the fact that currently it's leaking the underlying global object, which we don't want for security and object-identity reasons.

Bug: chromium:713732
Change-Id: I835ef510fc78f04c602434a7cec6420e027c4012
Reviewed-on: https://chromium-review.googlesource.com/520764
Commit-Queue: Toon Verwaest <verwaest@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45654}