Fix memory leak caused by treating Code::next_code_link as strong in marker.

src/mark-compact.cc

Index: src/mark-compact.cc
diff --git a/src/mark-compact.cc b/src/mark-compact.cc
index f5504478036709908462cd0832dd9fa369e5f570..4da38475de82fa4a98bc4a25eb9ee941ff1036a8 100644
--- a/src/mark-compact.cc
+++ b/src/mark-compact.cc
@@ -106,6 +106,10 @@ class VerifyMarkingVisitor: public ObjectVisitor {
     }
   }
 
+  WeakPointerMode weak_pointer_mode() {
+    return SKIP_WEAK_POINTERS;
+  }
+
  private:
   Heap* heap_;
 };
@@ -1860,6 +1864,10 @@ class RootMarkingVisitor : public ObjectVisitor {
     for (Object** p = start; p < end; p++) MarkObjectByPointer(p);
   }
 
+  WeakPointerMode weak_pointer_mode() {
+    return SKIP_WEAK_POINTERS;
+  }
+
  private:
   void MarkObjectByPointer(Object** p) {
     if (!(*p)->IsHeapObject()) return;