Add prototype transitions cache to Map.

src/objects-inl.h

Index: src/objects-inl.h
diff --git a/src/objects-inl.h b/src/objects-inl.h
index 2ed658c7ed4c3a7faddbc5db22d281d8d561cab7..e5caa073d48fbc6b97d841911e3465a08e5773ec 100644
--- a/src/objects-inl.h
+++ b/src/objects-inl.h
@@ -2537,6 +2537,12 @@ JSFunction* Map::unchecked_constructor() {
 }
 
 
+FixedArray* Map::unchecked_prototype_transitions() {
+  return reinterpret_cast<FixedArray*>(
+      READ_FIELD(this, kPrototypeTransitionsOffset));
+}
+
+
 Code::Flags Code::flags() {
   return static_cast<Flags>(READ_INT_FIELD(this, kFlagsOffset));
 }
@@ -2945,6 +2951,7 @@ MaybeObject* Map::GetSlowElementsMap() {
 ACCESSORS(Map, instance_descriptors, DescriptorArray,
           kInstanceDescriptorsOffset)
 ACCESSORS(Map, code_cache, Object, kCodeCacheOffset)
+ACCESSORS(Map, prototype_transitions, FixedArray, kPrototypeTransitionsOffset)
 ACCESSORS(Map, constructor, Object, kConstructorOffset)
 
 ACCESSORS(JSFunction, shared, SharedFunctionInfo, kSharedFunctionInfoOffset)