Update to use libbrillo Chrome proxy resolver

This uses the new proxy resolution code in libbrillo for doing proxy
resolution rather than making the D-Bus calls directly through the
generated bindings. The unittest for this class was removed since the
main function of it was removed. It now is essentially just a wrapper
around the call into libbrillo along with some minimal code for
allowing cancelling of callbacks.

BUG=chromium:771386
TEST=Unit tests pass

Change-Id: Ib0d4e413482f4bdfe0e9689f68118efbec94ec9d
Reviewed-on: https://chromium-review.googlesource.com/698964
Commit-Ready: Jeffrey Kardatzke <jkardatzke@google.com>
Tested-by: Jeffrey Kardatzke <jkardatzke@google.com>
Reviewed-by: Amin Hassani <ahassani@chromium.org>
Reviewed-by: Jeffrey Kardatzke <jkardatzke@google.com>
Reviewed-by: Sen Jiang <senj@chromium.org>
diff --git a/fake_system_state.cc b/fake_system_state.cc
index d51f775..9403f8b 100644
--- a/fake_system_state.cc
+++ b/fake_system_state.cc
@@ -21,7 +21,7 @@
 // Mock the SystemStateInterface so that we could lie that
 // OOBE is completed even when there's no such marker file, etc.
 FakeSystemState::FakeSystemState()
-    : mock_update_attempter_(this, nullptr, nullptr),
+    : mock_update_attempter_(this, nullptr),
       mock_request_params_(this),
       fake_update_manager_(&fake_clock_),
       clock_(&fake_clock_),