sync: Move unique_client_tag hashing code

sync/syncable/syncable_util.h

 // Copyright (c) 2012 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 SYNC_SYNCABLE_SYNCABLE_UTIL_H_
 #define SYNC_SYNCABLE_SYNCABLE_UTIL_H_
 
+#include <string>
 #include <vector>
 
 #include "base/basictypes.h"
+#include "sync/internal_api/public/base/model_type.h"
 
 namespace tracked_objects {
 class Location;
 }
 
 namespace syncer {
 namespace syncable {
 
 class BaseTransaction;
 class WriteTransaction;
 class MutableEntry;
 class Id;
 
 void ChangeEntryIDAndUpdateChildren(WriteTransaction* trans,
                                     MutableEntry* entry,
                                     const Id& new_id);
 
 bool IsLegalNewParent(BaseTransaction* trans, const Id& id, const Id& parentid);
 
 bool SyncAssert(bool condition,
                 const tracked_objects::Location& location,
                 const char* msg,
                 BaseTransaction* trans);
 
 int GetUnsyncedEntries(BaseTransaction* trans,
                        std::vector<int64> *handles);
 
+// Generates a fixed-length tag for the given string under the given model_type.
+std::string GenerateSyncableHash(
+    ModelType model_type, const std::string& client_tag);
+
 }  // namespace syncable
 }  // namespace syncer
 
 #endif  // SYNC_SYNCABLE_SYNCABLE_UTIL_H_