Implement identity hashes for proxies.

src/heap.h

Index: src/heap.h
diff --git a/src/heap.h b/src/heap.h
index 303df878337bff654640d99ad5344e49e13f74f1..787fbdcfe0cea817d2720ebbea05302f99a42eb6 100644
--- a/src/heap.h
+++ b/src/heap.h
@@ -451,6 +451,7 @@ class Heap {
   // size, but keeping the original prototype.  The receiver must have at least
   // the size of the new object.  The object is reinitialized and behaves as an
   // object that has been freshly allocated.
+  // Returns failure if an error occured, otherwise object.
   MUST_USE_RESULT MaybeObject* ReinitializeJSReceiver(JSReceiver* object,
                                                       InstanceType type,
                                                       int size);