[GCM] Add basic collapse key support for upstream

google_apis/gcm/engine/gcm_store_impl.h

Index: google_apis/gcm/engine/gcm_store_impl.h
diff --git a/google_apis/gcm/engine/gcm_store_impl.h b/google_apis/gcm/engine/gcm_store_impl.h
index 621bfe1008d78997106b54eeaa7a94719ef5cd8d..4f9c18815410dca9d69e9c92c1f1db97bca127c7 100644
--- a/google_apis/gcm/engine/gcm_store_impl.h
+++ b/google_apis/gcm/engine/gcm_store_impl.h
@@ -55,6 +55,10 @@ class GCM_EXPORT GCMStoreImpl : public GCMStore {
   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,