sync: Move unique_client_tag hashing code

sync/syncable/syncable_util.h

Index: sync/syncable/syncable_util.h
diff --git a/sync/syncable/syncable_util.h b/sync/syncable/syncable_util.h
index 425621e778fb3b6cf0529e6fc3b8edbc43275bf2..add3e2d066d594daabf4f1d1cf96c93d2fe4190e 100644
--- a/sync/syncable/syncable_util.h
+++ b/sync/syncable/syncable_util.h
@@ -5,9 +5,11 @@
 #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;
@@ -35,6 +37,10 @@ bool SyncAssert(bool condition,
 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