Background:

content:// URLs is mechanism on Android to access data from a content provider. The provider is named by the authority part of the URL, and it interprets the path.

Content providers are resolved within an ApplicationContext, so when considering OOP network service, I believe we need to keep its handling within the browser process.

(The failing tests we have rely on mock content providers, I am unclear where we use this in practice - supposedly downloads)

I believe we can use the same design as for file:// URLs -- adding a new URLLoaderFactory for content:// URLs to navigation and subresource requests.

(I have a CL for this, just waiting for approvals)

https://chromium-review.googlesource.com/c/chromium/src/+/1244101

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/801644121fc4c1b0ccfc05bb9bdb421cf4b8163a

commit 801644121fc4c1b0ccfc05bb9bdb421cf4b8163a
Author: Eric Roman <eroman@chromium.org>
Date: Tue Oct 02 22:13:54 2018

Support content:// URLs on Android using Network Service.

Bug:  882019 
Cq-Include-Trybots: luci.chromium.try:linux_mojo
Change-Id: I5c8f62c7615708e2b1019e11e9639552b9b267ae
Reviewed-on: https://chromium-review.googlesource.com/c/1244101
Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Reviewed-by: Ken Rockot <rockot@chromium.org>
Reviewed-by: Sami Kyöstilä <skyostil@chromium.org>
Commit-Queue: Eric Roman <eroman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#596004}
[modify] https://crrev.com/801644121fc4c1b0ccfc05bb9bdb421cf4b8163a/content/browser/BUILD.gn
[add] https://crrev.com/801644121fc4c1b0ccfc05bb9bdb421cf4b8163a/content/browser/android/content_url_loader_factory.cc
[add] https://crrev.com/801644121fc4c1b0ccfc05bb9bdb421cf4b8163a/content/browser/android/content_url_loader_factory.h
[modify] https://crrev.com/801644121fc4c1b0ccfc05bb9bdb421cf4b8163a/content/browser/android/url_request_content_job.h
[modify] https://crrev.com/801644121fc4c1b0ccfc05bb9bdb421cf4b8163a/content/browser/frame_host/render_frame_host_impl.cc
[modify] https://crrev.com/801644121fc4c1b0ccfc05bb9bdb421cf4b8163a/content/browser/loader/navigation_url_loader_impl.cc
[modify] https://crrev.com/801644121fc4c1b0ccfc05bb9bdb421cf4b8163a/testing/buildbot/filters/mojo.fyi.network_chrome_public_test_apk.filter