Add map-based read barrier to WeakCell

src/heap/heap.cc

Index: src/heap/heap.cc
diff --git a/src/heap/heap.cc b/src/heap/heap.cc
index 0742aafc83913c1c3e4a46792ca6da2e415c2ebe..d8e4b684f0fb85d431f2c53900ab1858382d8e1a 100644
--- a/src/heap/heap.cc
+++ b/src/heap/heap.cc
@@ -2702,7 +2702,8 @@ bool Heap::CreateInitialMaps() {
 
     ALLOCATE_MAP(CELL_TYPE, Cell::kSize, cell)
     ALLOCATE_MAP(PROPERTY_CELL_TYPE, PropertyCell::kSize, global_property_cell)
-    ALLOCATE_MAP(WEAK_CELL_TYPE, WeakCell::kSize, weak_cell)
+    ALLOCATE_MAP(WEAK_CELL_TYPE, WeakCell::kSize, used_weak_cell)
+    ALLOCATE_MAP(WEAK_CELL_TYPE, WeakCell::kSize, unused_weak_cell)
     ALLOCATE_MAP(FILLER_TYPE, kPointerSize, one_pointer_filler)
     ALLOCATE_MAP(FILLER_TYPE, 2 * kPointerSize, two_pointer_filler)
 
@@ -2839,7 +2840,7 @@ AllocationResult Heap::AllocateWeakCell(HeapObject* value) {
         AllocateRaw(size, OLD_POINTER_SPACE, OLD_POINTER_SPACE);
     if (!allocation.To(&result)) return allocation;
   }
-  result->set_map_no_write_barrier(weak_cell_map());
+  result->set_map_no_write_barrier(used_weak_cell_map());
   WeakCell::cast(result)->initialize(value);
   WeakCell::cast(result)->set_next(undefined_value(), SKIP_WRITE_BARRIER);
   return result;