[heap] Prevent leakage of GCCallbacksScope outside of heap.

src/heap/heap.cc

Index: src/heap/heap.cc
diff --git a/src/heap/heap.cc b/src/heap/heap.cc
index f1b8c474a81c486a816d500dd88870a336b0c2da..0e954ea059bfe266c1ba56cd7d573c6483cdda68 100644
--- a/src/heap/heap.cc
+++ b/src/heap/heap.cc
@@ -747,6 +747,20 @@ void Heap::PreprocessStackTraces() {
 }
 
 
+class GCCallbacksScope {
+ public:
+  explicit GCCallbacksScope(Heap* heap) : heap_(heap) {
+    heap_->gc_callbacks_depth_++;
+  }
+  ~GCCallbacksScope() { heap_->gc_callbacks_depth_--; }
+
+  bool CheckReenter() { return heap_->gc_callbacks_depth_ == 1; }
+
+ private:
+  Heap* heap_;
+};
+
+
 void Heap::HandleGCRequest() {
   if (incremental_marking()->request_type() ==
       IncrementalMarking::COMPLETE_MARKING) {