Add prototype transitions cache to Map.

src/heap.cc

Index: src/heap.cc
diff --git a/src/heap.cc b/src/heap.cc
index b1b510974090c14afb79a1908493bdc5e19d75b8..ac00c01c461e7646460410aad10732fb0b807163 100644
--- a/src/heap.cc
+++ b/src/heap.cc
@@ -1594,6 +1594,7 @@ MaybeObject* Heap::AllocateMap(InstanceType instance_type, int instance_size) {
   map->set_pre_allocated_property_fields(0);
   map->set_instance_descriptors(empty_descriptor_array());
   map->set_code_cache(empty_fixed_array());
+  map->set_prototype_transitions(empty_fixed_array());
   map->set_unused_property_fields(0);
   map->set_bit_field(0);
   map->set_bit_field2((1 << Map::kIsExtensible) | (1 << Map::kHasFastElements));
@@ -1686,12 +1687,15 @@ bool Heap::CreateInitialMaps() {
   // Fix the instance_descriptors for the existing maps.
   meta_map()->set_instance_descriptors(empty_descriptor_array());
   meta_map()->set_code_cache(empty_fixed_array());
+  meta_map()->set_prototype_transitions(empty_fixed_array());
 
   fixed_array_map()->set_instance_descriptors(empty_descriptor_array());
   fixed_array_map()->set_code_cache(empty_fixed_array());
+  fixed_array_map()->set_prototype_transitions(empty_fixed_array());
 
   oddball_map()->set_instance_descriptors(empty_descriptor_array());
   oddball_map()->set_code_cache(empty_fixed_array());
+  oddball_map()->set_prototype_transitions(empty_fixed_array());
 
   // Fix prototype object for existing maps.
   meta_map()->set_prototype(null_value());