[turbofan] Add support for extending properties backing store.

src/compiler/js-native-context-specialization.h

Index: src/compiler/js-native-context-specialization.h
diff --git a/src/compiler/js-native-context-specialization.h b/src/compiler/js-native-context-specialization.h
index 1373fa25fc3fb8b9b4f73891e5c95c8649e7bf35..cfbc8c6faccbb8330ad13a8aca7f126024369997 100644
--- a/src/compiler/js-native-context-specialization.h
+++ b/src/compiler/js-native-context-specialization.h
@@ -132,6 +132,10 @@ class JSNativeContextSpecialization final : public AdvancedReducer {
   Node* BuildCheckMaps(Node* receiver, Node* effect, Node* control,
                        std::vector<Handle<Map>> const& maps);
 
+  // Construct appropriate subgraph to extend properties backing store.
+  Node* BuildExtendPropertiesBackingStore(Handle<Map> map, Node* properties,
+                                          Node* effect, Node* control);
+
   // Adds stability dependencies on all prototypes of every class in
   // {receiver_type} up to (and including) the {holder}.
   void AssumePrototypesStable(std::vector<Handle<Map>> const& receiver_maps,