Make SyncEngineTest's SequencedWorkerPool multi-threaded.

chrome/browser/sync_file_system/drive_backend/sync_engine_unittest.cc

 // Copyright 2013 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 "chrome/browser/sync_file_system/drive_backend/sync_engine.h"
 
 #include <stddef.h>
 #include <utility>
 
 #include "base/files/scoped_temp_dir.h"
@@ skipping 14 matching lines @@
 #include "testing/gtest/include/gtest/gtest.h"
 
 namespace sync_file_system {
 namespace drive_backend {
 
 class SyncEngineTest : public testing::Test,
                        public base::SupportsWeakPtr<SyncEngineTest> {
  public:
   typedef RemoteFileSyncService::OriginStatusMap RemoteOriginStatusMap;
 
-  SyncEngineTest() : worker_pool_owner_(1, "Worker") {}
+  SyncEngineTest() : worker_pool_owner_(2, "Worker") {}
   ~SyncEngineTest() override {}
 
   void SetUp() override {
     ASSERT_TRUE(profile_dir_.CreateUniqueTempDir());
 
     std::unique_ptr<drive::DriveServiceInterface> fake_drive_service(
         new drive::FakeDriveService);
 
     scoped_refptr<base::SingleThreadTaskRunner> ui_task_runner =
         base::ThreadTaskRunnerHandle::Get();
@@ skipping 181 matching lines @@
 TEST_F(SyncEngineTest, ProcessRemoteChange) {
   SyncStatusCode sync_status;
   storage::FileSystemURL url;
   sync_engine()->ProcessRemoteChange(CreateResultReceiver(&sync_status, &url));
   WaitForWorkerTaskRunner();
   EXPECT_EQ(SYNC_STATUS_OK, sync_status);
 }
 
 }  // namespace drive_backend
 }  // namespace sync_file_system