Enable clang warning for ignored attributes.

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 {
+enum 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_