[GCM] Add basic collapse key support for upstream

google_apis/gcm/engine/gcm_store_impl.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 GOOGLE_APIS_GCM_ENGINE_GCM_STORE_IMPL_H_
 #define GOOGLE_APIS_GCM_ENGINE_GCM_STORE_IMPL_H_
 
 #include "base/basictypes.h"
 #include "base/memory/ref_counted.h"
 #include "base/memory/weak_ptr.h"
@@ skipping 37 matching lines @@
                                   const UpdateCallback& callback) OVERRIDE;
   virtual void RemoveIncomingMessage(const std::string& persistent_id,
                                      const UpdateCallback& callback) OVERRIDE;
   virtual void RemoveIncomingMessages(const PersistentIdList& persistent_ids,
                                       const UpdateCallback& callback) OVERRIDE;
 
   // Unacknowledged outgoing messages handling.
   virtual bool AddOutgoingMessage(const std::string& persistent_id,
                                   const MCSMessage& message,
                                   const UpdateCallback& callback) OVERRIDE;
+  virtual void OverwriteOutgoingMessage(const std::string& persistent_id,
+                                        const MCSMessage& message,
+                                        const UpdateCallback& callback)
+      OVERRIDE;
   virtual void RemoveOutgoingMessage(const std::string& persistent_id,
                                      const UpdateCallback& callback) OVERRIDE;
   virtual void RemoveOutgoingMessages(const PersistentIdList& persistent_ids,
                                       const UpdateCallback& callback) OVERRIDE;
 
   // User serial number handling.
   virtual void SetNextSerialNumber(int64 next_serial_number,
                                    const UpdateCallback& callback) OVERRIDE;
   virtual void AddUserSerialNumber(const std::string& username,
                                    int64 serial_number,
@@ skipping 32 matching lines @@
   scoped_refptr<base::SequencedTaskRunner> blocking_task_runner_;
 
   base::WeakPtrFactory<GCMStoreImpl> weak_ptr_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(GCMStoreImpl);
 };
 
 }  // namespace gcm
 
 #endif  // GOOGLE_APIS_GCM_ENGINE_GCM_STORE_IMPL_H_