Extensions: Add more test coverage for extensions ability to xhr file urls.

This CL adds more test coverage for an extensions ability to xhr file urls
through an extension frame. To XHR a file url an extension should have "Allow
access to file URLs" setting enabled and also have host permissions to file:///.
Tests are also added for activeTab.

This also helps discover a bug with the network service implementation of the
same.

BUG=816685

Cq-Include-Trybots: master.tryserver.chromium.linux:linux_mojo
Change-Id: Ic53a1ad9c25a5db56c69eb421734f76ba9f95a1e
Reviewed-on: https://chromium-review.googlesource.com/985798
Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Commit-Queue: Karan Bhatia <karandeepb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#547551}
13 files changed