[api] Add an API function to set the GetExternallyAllocatedMemoryInBytesCallback.

src/heap/heap.h

Index: src/heap/heap.h
diff --git a/src/heap/heap.h b/src/heap/heap.h
index d9ac1556b51dcf0c3466afa884a0004c640ec0c3..aaddcb9a5d6fefcb6829b5eca3c0f248e9495848 100644
--- a/src/heap/heap.h
+++ b/src/heap/heap.h
@@ -1175,6 +1175,14 @@ class Heap {
   // completes incremental marking in order to free external resources.
   void ReportExternalMemoryPressure();
 
+  typedef v8::Isolate::GetExternallyAllocatedMemoryInBytesCallback
+      GetExternallyAllocatedMemoryInBytesCallback;
+
+  void SetGetExternallyAllocatedMemoryInBytesCallback(
+      GetExternallyAllocatedMemoryInBytesCallback callback) {
+    external_memory_callback_ = callback;
+  }
+
   // Invoked when GC was requested via the stack guard.
   void HandleGCRequest();
 
@@ -1664,6 +1672,10 @@ class Heap {
     return (pretenure == TENURED) ? OLD_SPACE : NEW_SPACE;
   }
 
+  static size_t DefaultGetExternallyAllocatedMemoryInBytesCallback() {
+    return 0;
+  }
+
 #define ROOT_ACCESSOR(type, name, camel_name) \
   inline void set_##name(type* value);
   ROOT_LIST(ROOT_ACCESSOR)
@@ -2282,6 +2294,8 @@ class Heap {
   List<GCCallbackPair> gc_epilogue_callbacks_;
   List<GCCallbackPair> gc_prologue_callbacks_;
 
+  GetExternallyAllocatedMemoryInBytesCallback external_memory_callback_;
+
   int deferred_counters_[v8::Isolate::kUseCounterFeatureCount];
 
   GCTracer* tracer_;