Origin isolation: test initial about:blank

The initial about:blank inheriting its origin from the parent is a
rather special case, and deserves its own tests when origin isolation
is involved.

Bug: 1042415
Change-Id: I6c7d41e05b601bd1044f1c70f1ee7c75a44ffb14
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2255519


Commit-Queue: Domenic Denicola <domenic@chromium.org>
Reviewed-by: James MacLean <wjmaclean@chromium.org>
Cr-Commit-Position: refs/heads/master@{#780830}