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In order to avoid incompatible changes to RulesBasedHostResolverProc,
which is used all over the place (see crbug.com/878838), created
separate internal rules procs to handle those requests specifying
different sources.

Updated tests for the one piece of code currently specifying SYSTEM
source (via flag through the old API).

Bug: 846423
Change-Id: I1b92384d831579fe119817afc3e0cb92cfdf4565
Reviewed-on: https://chromium-review.googlesource.com/1195547
Commit-Queue: Eric Orth <ericorth@chromium.org>
Reviewed-by: Matt Menke <mmenke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#587746}