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..7974cf64abac7c34b421e83eb80217dca760d3aa 100644
--- a/src/objects-inl.h
+++ b/src/objects-inl.h
@@ -2036,7 +2036,7 @@ Object* WeakCell::value() const { return READ_FIELD(this, kValueOffset); }
 
 
 void WeakCell::clear() {
-  DCHECK(GetHeap()->gc_state() == Heap::MARK_COMPACT);
+  DCHECK(GetHeap()->gc_state() == Heap::MARK_COMPACT || value()->IsUndefined());

[Michael Starzinger, 2015/11/27 10:06:40]

I assume this exception is just here because of the call to "clear" in
Heap::CreateInitialObjects. Correct? Could we instead check "this ==
GetHeap()->empty_weak_cell()" to make that clearer and keep the filter of what
is allowed narrow?

[mvstanton, 2015/12/01 11:28:15]

Good idea, done.

   WRITE_FIELD(this, kValueOffset, Smi::FromInt(0));
 }