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This CL migrates ContentHashFetcher away from URLRequestFetcher to
SimpleURLLoader.

In order to accomplish this, a few other related changes are also performed:

1) The regular flow of the code is ContentVerifier -> ContentHash ->
   ContentHashFetcher. In ContentVerifier, for instance, UI, IO and
   "File tasks" threads take place.
   Previously, the URLRequestFetcher logic residing in ContentHashFetcher
   executed in the "file tasks" thread.

   As part of the migration from URLRequestFetcher to SimpleURLLoader
   machinery, this CL also changes the ContentHashFetcher logic to
   execute to the IO thread.
   Note that it could be possible to keep the new SimpleURLLoader logic
   in the "file tasks" thread. However, this would impose a way bigger change,
   and require unittests to be considerably rewritten as well (see for
   patchsets 7, 8 and 9.
   The issue is that mojo objects are not thread safe. We could create a new
   URLLoaderFactory for the extensions thread, but then we'd need to create a
   new one when the network service crashes (Something this SharedURLLoaderFactory
   one, already bound to the IOThread, magically handles), but that would be
   significantly more complicated.

2) content_verifier_hash_fetch_behavior_browsertest.cc was changed
   to support running with the network service feature both enabled
   and disabled. This is a pattern that is also present in various other
   unittests.

In summary, the migration to use SimpleURLLoader performed here includes
also a change in the thread that ContentHashFetcher runs on, but no
functionality change is expected from this CL.

BUG=773295,844926

Change-Id: If570f69d01ff75ac59d8d043f8687621336dddcf
Reviewed-on: https://chromium-review.googlesource.com/1056587
Commit-Queue: Antonio Gomes <tonikitoo@igalia.com>
Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Reviewed-by: Matt Menke <mmenke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#561480}