Introduce EmbedderHeapTracer

src/heap/heap.h

Index: src/heap/heap.h
diff --git a/src/heap/heap.h b/src/heap/heap.h
index 8af1e3844aba9ec14264d7c11e93b100a1602ace..61ec22874d8490e14edada2b1fe4065be28bfabf 100644
--- a/src/heap/heap.h
+++ b/src/heap/heap.h
@@ -913,6 +913,7 @@ class Heap {
   // ===========================================================================
 
   GCTracer* tracer() { return tracer_; }
+  EmbedderHeapTracer* embedder_heap_tracer() { return embedder_heap_tracer_; }
 
   PromotionQueue* promotion_queue() { return &promotion_queue_; }
 
@@ -1310,6 +1311,9 @@ class Heap {
   void CallGCPrologueCallbacks(GCType gc_type, GCCallbackFlags flags);
   void CallGCEpilogueCallbacks(GCType gc_type, GCCallbackFlags flags);
 
+  void SetEmbedderHeapTracer(EmbedderHeapTracer* tracer);
+  void UnsetEmbedderHeapTracer();
+
   // ===========================================================================
   // Allocation methods. =======================================================
   // ===========================================================================
@@ -2077,8 +2081,8 @@ class Heap {
 
   Object* encountered_transition_arrays_;
 
-  List<GCCallbackPair> gc_epilogue_callbacks_;
   List<GCCallbackPair> gc_prologue_callbacks_;
+  List<GCCallbackPair> gc_epilogue_callbacks_;
 
   // Total RegExp code ever generated
   double total_regexp_code_generated_;
@@ -2086,6 +2090,7 @@ class Heap {
   int deferred_counters_[v8::Isolate::kUseCounterFeatureCount];
 
   GCTracer* tracer_;
+  EmbedderHeapTracer* embedder_heap_tracer_;
 
   int high_survival_rate_period_length_;
   intptr_t promoted_objects_size_;