Introduce instance type for transition arrays.

src/heap/heap.cc

Index: src/heap/heap.cc
diff --git a/src/heap/heap.cc b/src/heap/heap.cc
index 5137301ddf46bc324da3c5784959085fefbabab3..2f017c2c0bdc8f004f3372ccde358ad2613fe365 100644
--- a/src/heap/heap.cc
+++ b/src/heap/heap.cc
@@ -180,6 +180,7 @@ Heap::Heap()
   set_allocation_sites_list(Smi::FromInt(0));
   set_encountered_weak_collections(Smi::FromInt(0));
   set_encountered_weak_cells(Smi::FromInt(0));
+  set_encountered_transition_arrays(Smi::FromInt(0));
   // Put a dummy entry in the remembered pages so we can find the list the
   // minidump even if there are no real unmapped pages.
   RememberUnmappedPage(NULL, false);
@@ -2333,6 +2334,7 @@ bool Heap::CreateInitialMaps() {
     ALLOCATE_MAP(FILLER_TYPE, kPointerSize, one_pointer_filler)
     ALLOCATE_MAP(FILLER_TYPE, 2 * kPointerSize, two_pointer_filler)
 
+    ALLOCATE_VARSIZE_MAP(TRANSITION_ARRAY_TYPE, transition_array)
 
     for (unsigned i = 0; i < arraysize(struct_table); i++) {
       const StructTable& entry = struct_table[i];
@@ -2494,6 +2496,21 @@ AllocationResult Heap::AllocateWeakCell(HeapObject* value) {
 }
 
 
+AllocationResult Heap::AllocateTransitionArray(int capacity) {
+  DCHECK(capacity > 0);
+  HeapObject* raw_array = nullptr;
+  {
+    AllocationResult allocation = AllocateRawFixedArray(capacity, TENURED);
+    if (!allocation.To(&raw_array)) return allocation;
+  }
+  raw_array->set_map_no_write_barrier(transition_array_map());
+  TransitionArray* array = TransitionArray::cast(raw_array);
+  array->set_length(capacity);
+  MemsetPointer(array->data_start(), undefined_value(), capacity);
+  return array;
+}
+
+
 void Heap::CreateApiObjects() {
   HandleScope scope(isolate());
   Factory* factory = isolate()->factory();