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 597a00cd6c084b6e0c7ddc066a1fef7796671108..3695b2fff001aef9ca5bb97349e477e889ab6aac 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));
 }