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This is a follow up of [1], where the sync logic switched away from
using URLFetcher, in favor of SimpleURLLoader (network s13n effort).

[1] https://crrev.com/c/1174655

As part of this migration, sync_client (the executable) switched to
use TestSharedURLLoaderFactory. However,
TransitionalURLLoaderFactoryOwner was particularly implemented to be
used for standalong applications, which is the case of sync_client.

This CL switches sync_client (target components/sync/tools:sync_client)
to use it.

BUG=844968

Change-Id: I9eaa980c3b211232b897e6aa8bb6336f5e35ec10
Reviewed-on: https://chromium-review.googlesource.com/1222508Reviewed-by: Maks Orlovich <morlovich@chromium.org>
Reviewed-by: Marc Treib <treib@chromium.org>
Commit-Queue: Antonio Gomes <tonikitoo@igalia.com>
Cr-Commit-Position: refs/heads/master@{#590984}