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This complements the existing user policy and adds print servers defined
by device policy. The device policy has its own corresponding whitelist
and if there is existing print servers in user policy also in device
policy, then the user policy print servers take precedence (which is
only for the purpose of the print server names currently).
Also changed to using shared url loader factory since the
SystemNetworkContext is non-unit-testable.

Bug: 1100777
Change-Id: I0e45597437b59b5eb940c4fbf5fb63596b8504dd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2293178Reviewed-by: Lei Zhang <thestig@chromium.org>
Reviewed-by: Luum Habtemariam <luum@chromium.org>
Reviewed-by: Piotr Pawliczek <pawliczek@chromium.org>
Commit-Queue: Matt Menard <mattme@google.com>
Cr-Commit-Position: refs/heads/master@{#800322}