mac: build fix after http://crrev.com/1364803004

snapshot/mac/thread_snapshot_mac.cc

 // Copyright 2014 The Crashpad Authors. All rights reserved.
 //
 // Licensed under the Apache License, Version 2.0 (the "License");
 // you may not use this file except in compliance with the License.
 // You may obtain a copy of the License at
 //
 //     http://www.apache.org/licenses/LICENSE-2.0
 //
 // Unless required by applicable law or agreed to in writing, software
 // distributed under the License is distributed on an "AS IS" BASIS,
@@ skipping 93 matching lines @@
 int ThreadSnapshotMac::Priority() const {
   INITIALIZATION_STATE_DCHECK_VALID(initialized_);
   return priority_;
 }
 
 uint64_t ThreadSnapshotMac::ThreadSpecificDataAddress() const {
   INITIALIZATION_STATE_DCHECK_VALID(initialized_);
   return thread_specific_data_address_;
 }
 
+std::vector<const MemorySnapshot*> ThreadSnapshotMac::ExtraMemory() const {
+  return std::vector<const MemorySnapshot*>();
+}
+
 }  // namespace internal
 }  // namespace crashpad