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

src/heap/heap.cc

Index: src/heap/heap.cc
diff --git a/src/heap/heap.cc b/src/heap/heap.cc
index d7f25874295d0f14c16a9ac149b3b7f824562255..b6046444529a6c929caf16393dd2b4a76fa7807a 100644
--- a/src/heap/heap.cc
+++ b/src/heap/heap.cc
@@ -381,6 +381,8 @@ void Heap::PrintShortHeapStatistics() {
                this->CommittedMemory() / KB);
   PrintIsolate(isolate_, "External memory reported: %6" PRId64 " KB\n",
                external_memory_ / KB);
+  PrintIsolate(isolate_, "External memory global %zu KB\n",
+               external_memory_callback_() / KB);
   PrintIsolate(isolate_, "Total time spent in GC  : %.1f ms\n",
                total_gc_time_ms_);
 }
@@ -964,7 +966,6 @@ void Heap::ReportExternalMemoryPressure() {
   }
 }
 
-
 void Heap::EnsureFillerObjectAtTop() {
   // There may be an allocation memento behind objects in new space. Upon
   // evacuation of a non-full new space (or if we are on the last page) there
@@ -5739,6 +5740,9 @@ bool Heap::SetUp() {
       *this, ScavengeJob::kBytesAllocatedBeforeNextIdleTask);
   new_space()->AddAllocationObserver(idle_scavenge_observer_);
 
+  SetGetExternallyAllocatedMemoryInBytesCallback(
+      DefaultGetExternallyAllocatedMemoryInBytesCallback);
+
   return true;
 }