[heap] Fix page promotions tests

src/heap/mark-compact.h

Index: src/heap/mark-compact.h
diff --git a/src/heap/mark-compact.h b/src/heap/mark-compact.h
index d54469a8a7bdd82c2348384fc0212154ceaf6778..2ebf4a720c19a5dd0e55779584f6d26039c7ab86 100644
--- a/src/heap/mark-compact.h
+++ b/src/heap/mark-compact.h
@@ -423,10 +423,11 @@ class MinorMarkCompactCollector {
 
   void CollectGarbage();
 
+  inline Heap* heap() const { return heap_; }
+
  private:
   class RootMarkingVisitor;
 
-  inline Heap* heap() { return heap_; }
   inline Isolate* isolate() { return heap()->isolate(); }
   inline MarkingDeque* marking_deque() { return &marking_deque_; }