Made iteration of global handles more efficient on scavenges.

src/heap.h

Index: src/heap.h
diff --git a/src/heap.h b/src/heap.h
index 285260594f373230322891b14a22d936456e1a5c..a7b8f44b3281400ee0a79b2f0e0ce77f2bb81024 100644
--- a/src/heap.h
+++ b/src/heap.h
@@ -729,9 +729,9 @@ class Heap : public AllStatic {
   static String* hidden_symbol() { return hidden_symbol_; }
 
   // Iterates over all roots in the heap.
-  static void IterateRoots(ObjectVisitor* v);
+  static void IterateRoots(ObjectVisitor* v, VisitMode mode);
   // Iterates over all strong roots in the heap.
-  static void IterateStrongRoots(ObjectVisitor* v);
+  static void IterateStrongRoots(ObjectVisitor* v, VisitMode mode);
 
   // Iterates remembered set of an old space.
   static void IterateRSet(PagedSpace* space, ObjectSlotCallback callback);