Remove MessageLoop references in services/network/public/cpp

MessageLoop is going away soon.

This CL was uploaded by git cl split.

R=dominickn@chromium.org

Bug: 891670
Change-Id: Ied8aaf4fc0778567283544e64c84b37f3f547c5e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1878373
Commit-Queue: Dominick Ng <dominickn@chromium.org>
Reviewed-by: Dominick Ng <dominickn@chromium.org>
Auto-Submit: Carlos Caballero <carlscab@google.com>
Cr-Commit-Position: refs/heads/master@{#709234}

services/network/public/cpp/network_mojom_traits_unittest.cc

@@ -2,8 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-#include "base/message_loop/message_loop.h"
 #include "base/strings/string_util.h"
+#include "base/test/task_environment.h"
 #include "mojo/public/cpp/bindings/pending_remote.h"
 #include "mojo/public/cpp/bindings/receiver_set.h"
 #include "mojo/public/cpp/bindings/remote.h"
@@ -34,7 +34,7 @@ class NetworkStructTraitsTest : public testing::Test,
     std::move(callback).Run(header);
   }
 
-  base::MessageLoop loop_;
+  base::test::SingleThreadTaskEnvironment task_environment_;
   mojo::ReceiverSet<TraitsTestService> traits_test_receivers_;
   DISALLOW_COPY_AND_ASSIGN(NetworkStructTraitsTest);
 };