Revert r19897, r19898 for breaking mac-64 debug.

src/mark-compact.cc

Index: src/mark-compact.cc
diff --git a/src/mark-compact.cc b/src/mark-compact.cc
index 9a0a1eafcbf9fa6c08094601886a9897750c76ef..810f25d439fb3e11e5e351c035141929b855a99d 100644
--- a/src/mark-compact.cc
+++ b/src/mark-compact.cc
@@ -1860,10 +1860,6 @@ class RootMarkingVisitor : public ObjectVisitor {
     for (Object** p = start; p < end; p++) MarkObjectByPointer(p);
   }
 
-  // Skip the weak next code link in a code object, which is visited in
-  // ProcessTopOptimizedFrame.
-  void VisitNextCodeLink(Object** p) { }
-
  private:
   void MarkObjectByPointer(Object** p) {
     if (!(*p)->IsHeapObject()) return;