[SyncFS][Reland] Relay callback destruction to TaskRunner on RelayCallbackToTaskRunner

chrome/browser/sync_file_system/drive_backend/drive_backend_sync_unittest.cc

 // Copyright 2014 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 <algorithm>
 #include <stack>
 
 #include "base/file_util.h"
 #include "base/message_loop/message_loop.h"
 #include "base/run_loop.h"
@@ skipping 113 matching lines @@
     for (iterator itr = file_systems_.begin();
          itr != file_systems_.end(); ++itr) {
       itr->second->TearDown();
       delete itr->second;
     }
     file_systems_.clear();
 
     fake_drive_service_helper_.reset();
     remote_sync_service_.reset();
 
+    content::BrowserThread::GetBlockingPool()->FlushForTesting();
     base::RunLoop().RunUntilIdle();
     RevokeSyncableFileSystem();
   }
 
   virtual void OnRemoteChangeQueueUpdated(int64 pending_changes_hint) OVERRIDE {
     pending_remote_changes_ = pending_changes_hint;
   }
 
   virtual void OnLocalChangeAvailable(int64 pending_changes_hint) OVERRIDE {
     pending_local_changes_ = pending_changes_hint;
@@ skipping 1465 matching lines @@
 
   EXPECT_EQ(1u, CountApp());
   EXPECT_EQ(1u, CountLocalFile(app_id));
 
   EXPECT_EQ(2u, CountMetadata());
   EXPECT_EQ(2u, CountTracker());
 }
 
 }  // namespace drive_backend
 }  // namespace sync_file_system