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This is a reland of dccf844a

This was reverted because NetworkContextConfigurationBrowserTests
were failing on Mac due to crbug.com/843324. This is because those
tests call AddSimulatedFailure() on the host resolver, which now
sends those rules to NetworkServiceTest (which doesn't work on Mac).
I disabled the NetworkContextConfigurationBrowserTests on Mac that
run with network service enabled.

Original change's description:
> Convert dns_api.cc to mojo host resolver
>
> Bug: 874658
> Cq-Include-Trybots: luci.chromium.try:linux_mojo
> Change-Id: I576d2877ec71df8e489d349916f758ba6fe74263
> Reviewed-on: https://chromium-review.googlesource.com/1176725
> Reviewed-by: Will Harris <wfh@chromium.org>
> Reviewed-by: John Abd-El-Malek <jam@chromium.org>
> Commit-Queue: Clark DuVall <cduvall@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#583878}

TBR=wfh@chromium.org,jam@chromium.org

Bug: 874658
Change-Id: Ib26c612a450a48818a10f817bd2d309292d4c9ea
Cq-Include-Trybots: luci.chromium.try:linux_mojo
Reviewed-on: https://chromium-review.googlesource.com/1179922Reviewed-by: Clark DuVall <cduvall@chromium.org>
Reviewed-by: Matt Menke <mmenke@chromium.org>
Commit-Queue: Clark DuVall <cduvall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#584122}