Use WeakCells in the optimized code map rather than traversing in pause.

src/objects-inl.h

Index: src/objects-inl.h
diff --git a/src/objects-inl.h b/src/objects-inl.h
index 4033f886a065980213c6442924390117f13e708f..e848798310aef82ef96e646a6a99408d178c5354 100644
--- a/src/objects-inl.h
+++ b/src/objects-inl.h
@@ -2036,7 +2036,10 @@ Object* WeakCell::value() const { return READ_FIELD(this, kValueOffset); }
 
 
 void WeakCell::clear() {
-  DCHECK(GetHeap()->gc_state() == Heap::MARK_COMPACT);
+  // Either the garbage collector is clearing the cell or we are simply
+  // initializing the root empty weak cell.
+  DCHECK(GetHeap()->gc_state() == Heap::MARK_COMPACT ||
+         this == GetHeap()->empty_weak_cell());
   WRITE_FIELD(this, kValueOffset, Smi::FromInt(0));
 }