Add browser tests for HTTP auth and DNS stub resolver configuration.

These aren't full integration tests - they don't make sure that the
correct configuration is sent to the network service on start, or that
the configuration is updated as the prefs change. Instead, they just
check the code that maps prefs to NetworkService configuration structs.

Bug: 881168
Change-Id: I923d8536686ca3a10083c1b8ba0b76c5703e1f9d
Reviewed-on: https://chromium-review.googlesource.com/1210365


Reviewed-by: Asanka Herath <asanka@chromium.org>
Commit-Queue: Matt Menke <mmenke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#590481}