[Arguments] Introduce a new backing store for non-strict arguments objects.

src/heap.cc

Index: src/heap.cc
diff --git a/src/heap.cc b/src/heap.cc
index 9a5a2b05a72a5699fed20e306307e70ea13dace6..6939293baba676db83dfde9f35f727d980cdb73e 100644
--- a/src/heap.cc
+++ b/src/heap.cc
@@ -1730,6 +1730,12 @@ bool Heap::CreateInitialMaps() {
   }
   set_external_float_array_map(Map::cast(obj));
 
+  { MaybeObject* maybe_obj =
+        AllocateMap(FIXED_ARRAY_TYPE, kVariableSizeSentinel);
+    if (!maybe_obj->ToObject(&obj)) return false;
+  }
+  set_non_strict_arguments_elements_map(Map::cast(obj));
+
   { MaybeObject* maybe_obj = AllocateMap(CODE_TYPE, kVariableSizeSentinel);
     if (!maybe_obj->ToObject(&obj)) return false;
   }