Use ScopedTaskEnvironment instead of MessageLoopForUI in ash tests.

ash/test/ash_test_environment_default.cc

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "ash/test/ash_test_environment.h"
 
 #include "ash/test/ash_test_views_delegate.h"
 #include "base/memory/ptr_util.h"
-#include "base/message_loop/message_loop.h"
 #include "base/run_loop.h"
+#include "base/test/scoped_task_environment.h"
 #include "base/test/sequenced_worker_pool_owner.h"
 #include "base/threading/sequenced_worker_pool.h"
 
 namespace ash {
 namespace test {
 namespace {
 
 class AshTestEnvironmentDefault : public AshTestEnvironment {
  public:
-  AshTestEnvironmentDefault() {}
+  AshTestEnvironmentDefault()
+      : scoped_task_environment_(
+            base::test::ScopedTaskEnvironment::MainThreadType::UI) {}
 
   ~AshTestEnvironmentDefault() override {
     base::RunLoop().RunUntilIdle();
     blocking_pool_owner_.reset();
     base::RunLoop().RunUntilIdle();
   }
 
   // AshTestEnvironment:
   base::SequencedWorkerPool* GetBlockingPool() override {
     if (!blocking_pool_owner_) {
       const size_t kMaxNumberThreads = 3u;  // Matches that of content.
       const char kThreadNamePrefix[] = "AshBlocking";
       blocking_pool_owner_ = base::MakeUnique<base::SequencedWorkerPoolOwner>(
           kMaxNumberThreads, kThreadNamePrefix);
     }
     return blocking_pool_owner_->pool().get();
   }
   std::unique_ptr<AshTestViewsDelegate> CreateViewsDelegate() override {
     return base::MakeUnique<AshTestViewsDelegate>();
   }
 
  private:
-  base::MessageLoopForUI message_loop_;
+  base::test::ScopedTaskEnvironment scoped_task_environment_;
   std::unique_ptr<base::SequencedWorkerPoolOwner> blocking_pool_owner_;
 
   DISALLOW_COPY_AND_ASSIGN(AshTestEnvironmentDefault);
 };
 
 }  // namespace
 
 // static
 std::unique_ptr<AshTestEnvironment> AshTestEnvironment::Create() {
   return base::MakeUnique<AshTestEnvironmentDefault>();
 }
 
 // static
 std::string AshTestEnvironment::Get100PercentResourceFileName() {
   return "ash_test_resources_100_percent.pak";
 }
 
 }  // namespace test
 }  // namespace ash