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ProxyResolvingClientSocket accesses socket pools, http auth cache,
client certs cache directly.  As a part of the network servicification
effort, all interactions with //net need to be done using mojo APIs.
Socket usage will also be through mojo APIs. This CL moves this class to
services/network so the API surface that we need to export is smaller.

This CL:
- Moves ProxyResolvingClientSocket to services/network.
- Adds a few additional unit tests.

See design doc:  https://docs.google.com/document/d/1iQl_Y2o7vykiPXpZiNbKov-WZbUb4RprmXIk311tTso/edit?usp=sharing.

Bug: 721401

Cq-Include-Trybots: master.tryserver.chromium.android:android_cronet_tester;master.tryserver.chromium.mac:ios-simulator-cronet
Change-Id: Idbc0cb2e3a9713ef111ddd8b5845f27b1445574e
Reviewed-on: https://chromium-review.googlesource.com/763651
Commit-Queue: Helen Li <xunjieli@chromium.org>
Reviewed-by: Matt Menke <mmenke@chromium.org>
Reviewed-by: Sergey Ulanov <sergeyu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#526510}