[Sync] Refactor sync manager into interface.

sync/internal_api/public/util/sync_string_conversions.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_INTERNAL_API_PUBLIC_UTIL_SYNC_STRING_CONVERSIONS_H_
+#define SYNC_INTERNAL_API_PUBLIC_UTIL_SYNC_STRING_CONVERSIONS_H_
+
+#include "sync/internal_api/public/sync_manager.h"
+
+namespace syncer {
+
+const char* ConnectionStatusToString(ConnectionStatus status);
+
+// Returns the string representation of a PassphraseRequiredReason value.
+const char* PassphraseRequiredReasonToString(
+    PassphraseRequiredReason reason);
+
+}
+
+#endif  // SYNC_INTERNAL_API_PUBLIC_UTIL_SYNC_STRING_CONVERSIONS_H_