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URLFetcher will stop working with advent of Network Service, and
SimpleURLLoader is the replacement API for most clients.
This CL migrates chromeos::chromeos::AttestationCAClient and the
respective unittests away from URLFetcher.

BUG=773295,879782

Change-Id: I9ccdad3a73108ad3e3e7ab4f148b93048433b3bd
Reviewed-on: https://chromium-review.googlesource.com/1202223
Commit-Queue: Antonio Gomes <tonikitoo@igalia.com>
Reviewed-by: Matt Menke <mmenke@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#590089}