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

src/heap.cc

Index: src/heap.cc
diff --git a/src/heap.cc b/src/heap.cc
index 42e56ca1eb9e2bb40498d9ea2d341c71a67f6b4e..638e10ca0fecc4d44cadd2a949fb6d771898ae3f 100644
--- a/src/heap.cc
+++ b/src/heap.cc
@@ -7096,6 +7096,10 @@ class PathTracer::MarkVisitor: public ObjectVisitor {
     }
   }
 
+  WeakPointerMode weak_pointer_mode() {
+    return SKIP_WEAK_POINTERS;
+  }
+
  private:
   PathTracer* tracer_;
 };
@@ -7112,6 +7116,10 @@ class PathTracer::UnmarkVisitor: public ObjectVisitor {
     }
   }
 
+  WeakPointerMode weak_pointer_mode() {
+    return SKIP_WEAK_POINTERS;
+  }
+
  private:
   PathTracer* tracer_;
 };