Make `content` target build without unused data members on Linux.

content/renderer/background_sync/background_sync_client_impl.h

 // Copyright 2015 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 CONTENT_RENDERER_BACKGROUND_SYNC_BACKGROUND_SYNC_CLIENT_IMPL_H_
 #define CONTENT_RENDERER_BACKGROUND_SYNC_BACKGROUND_SYNC_CLIENT_IMPL_H_
 
 #include <stdint.h>
 
 #include <map>
@@ skipping 24 matching lines @@
   // BackgroundSyncServiceClient methods:
   void Sync(int64_t handle_id,
             content::BackgroundSyncEventLastChance last_chance,
             const SyncCallback& callback) override;
   void SyncDidGetRegistration(
       int64_t callback_id,
       content::BackgroundSyncEventLastChance last_chance,
       BackgroundSyncError error,
       SyncRegistrationPtr registration);
 
-  int64_t service_worker_registration_id_;
   mojo::StrongBinding<BackgroundSyncServiceClient> binding_;
 
   int64_t callback_seq_num_;
   std::map<int64_t, SyncCallback> sync_callbacks_;
 
   DISALLOW_COPY_AND_ASSIGN(BackgroundSyncClientImpl);
 };
 
 }  // namespace content
 
 #endif  // CONTENT_RENDERER_BACKGROUND_SYNC_BACKGROUND_SYNC_CLIENT_IMPL_H_