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 e1e3e48f9b5c1e847a6b650ea1ecdd88118cef8d..ae7f1c864e667436c37282e485eaefe39d4ca9d2 100644
--- a/src/objects-inl.h
+++ b/src/objects-inl.h
@@ -2029,7 +2029,10 @@ 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));
 }