Extensions: Add test coverage for an extensions ability to embed file iframes.

This CL adds test coverage regarding an extension frame's ability to embed a
file frame. This ensures than an extension can only embed a file frame if it has
explicit file access and host permissions to the file scheme.

BUG=816685

Cq-Include-Trybots: master.tryserver.chromium.linux:linux_mojo
Change-Id: I8d167d9a8629ffece29d44bee4bd6db2ef988b02
Reviewed-on: https://chromium-review.googlesource.com/1003762
Commit-Queue: Karan Bhatia <karandeepb@chromium.org>
Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#551267}