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

sync/internal_api/public/shutdown_reason.h

Index: sync/internal_api/public/shutdown_reason.h
diff --git a/sync/internal_api/public/shutdown_reason.h b/sync/internal_api/public/shutdown_reason.h
new file mode 100644
index 0000000000000000000000000000000000000000..60060744b40be65160eeae034cc8ec8d07705f7b
--- /dev/null
+++ b/sync/internal_api/public/shutdown_reason.h
@@ -0,0 +1,23 @@
+// 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_