Wire sync shutdown reason from PSS all the way down to sync manager.

sync/internal_api/public/shutdown_reason.h

+// 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.
+
+#ifndef SYNC_INTERNAL_API_PUBLIC_SHUTDOWN_REASON_H_
+#define SYNC_INTERNAL_API_PUBLIC_SHUTDOWN_REASON_H_
+
+#include "sync/base/sync_export.h"
+
+namespace syncer {
+
+// Reason for shutting down sync engine.
+enum SYNC_EXPORT ShutdownReason {
+  STOP_SYNC,         // Sync is asked to stop, e.g. due to scarce resource.
+  DISABLE_SYNC,      // Sync is disabled, e.g. user sign out, dashboard clear.
+  BROWSER_SHUTDOWN,  // Browser is closed.
+  SWITCH_MODE_SYNC,  // Engine is shut down and a new engine will start in
+                     // sync mode.
+};
+
+}  // namespace syncer
+
+#endif  // SYNC_INTERNAL_API_PUBLIC_SHUTDOWN_REASON_H_