[scheduling] Use ScopedTaskEnvironment instead of MessageLoop in proxy_config

MessageLoop will go away, eventually.

BUG=891670

Change-Id: I23c823f9391b84eb3cb97143bf0482dba64f9101
Reviewed-on: https://chromium-review.googlesource.com/c/1352424Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Commit-Queue: Carlos Caballero <carlscab@google.com>
Cr-Commit-Position: refs/heads/master@{#611748}

components/proxy_config/pref_proxy_config_tracker_impl_unittest.cc

@@ -8,8 +8,8 @@
 #include <string>
 
 #include "base/files/file_path.h"
-#include "base/message_loop/message_loop.h"
 #include "base/run_loop.h"
+#include "base/test/scoped_task_environment.h"
 #include "base/threading/thread_task_runner_handle.h"
 #include "components/prefs/pref_registry_simple.h"
 #include "components/prefs/testing_pref_service.h"
@@ -104,7 +104,7 @@ class PrefProxyConfigTrackerImplTest : public testing::Test {
     proxy_config_service_.reset();
   }
 
-  base::MessageLoop loop_;
+  base::test::ScopedTaskEnvironment task_environment_;
   std::unique_ptr<TestingPrefServiceSimple> pref_service_;
   TestProxyConfigService* delegate_service_; // weak
   std::unique_ptr<net::ProxyConfigService> proxy_config_service_;