Revert "Migrate ContentHashFetcher to SimpleURLLoader"

This reverts commit 27f8891c7cd9a86b2c92508cf6818726df4058d6.

Reason for revert: suspect for crbug.com/846417

Original change's description:
> Migrate ContentHashFetcher to SimpleURLLoader
> 
> 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}

TBR=lazyboy@chromium.org,rdevlin.cronin@chromium.org,mmenke@chromium.org,tonikitoo@igalia.com

Change-Id: I31235cf26cfff34aac0ff1e4047575ed2881722b
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 773295, 844926, 846417
Reviewed-on: https://chromium-review.googlesource.com/1072110
Reviewed-by: Gabriel Charette <gab@chromium.org>
Commit-Queue: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#561572}
10 files changed