Explicitly check various sync_file_system classes live on the IO thread.

chrome/browser/sync_file_system/local/syncable_file_system_operation.h

 // 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.
 
 #ifndef CHROME_BROWSER_SYNC_FILE_SYSTEM_LOCAL_SYNCABLE_FILE_SYSTEM_OPERATION_H_
 #define CHROME_BROWSER_SYNC_FILE_SYSTEM_LOCAL_SYNCABLE_FILE_SYSTEM_OPERATION_H_
 
 #include <stdint.h>
 
 #include <memory>
 #include <vector>
 
 #include "base/callback.h"
 #include "base/macros.h"
 #include "base/memory/ref_counted.h"
 #include "base/memory/weak_ptr.h"
-#include "base/threading/non_thread_safe.h"
 #include "storage/browser/fileapi/file_system_operation.h"
 #include "storage/browser/fileapi/file_system_url.h"
 
 namespace storage {
 class FileSystemContext;
 class FileSystemOperationContext;
 }
 
 namespace sync_file_system {
 
 class SyncableFileOperationRunner;
 
 // A wrapper class of FileSystemOperation for syncable file system.
 class SyncableFileSystemOperation
-    : public NON_EXPORTED_BASE(storage::FileSystemOperation),
-      public base::NonThreadSafe {
+    : public NON_EXPORTED_BASE(storage::FileSystemOperation) {
  public:
   ~SyncableFileSystemOperation() override;
 
   // storage::FileSystemOperation overrides.
   void CreateFile(const storage::FileSystemURL& url,
                   bool exclusive,
                   const StatusCallback& callback) override;
   void CreateDirectory(const storage::FileSystemURL& url,
                        bool exclusive,
                        bool recursive,
@@ skipping 85 matching lines @@
   StatusCallback completion_callback_;
 
   base::WeakPtrFactory<SyncableFileSystemOperation> weak_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(SyncableFileSystemOperation);
 };
 
 }  // namespace sync_file_system
 
 #endif  // CHROME_BROWSER_SYNC_FILE_SYSTEM_LOCAL_SYNCABLE_FILE_SYSTEM_OPERATION_H_