[heap] Properly decrement amount of externally allocated memory

src/heap/heap.cc

Index: src/heap/heap.cc
diff --git a/src/heap/heap.cc b/src/heap/heap.cc
index a4357f11fd2810df88382b46e6482cedf957d5af..086f5cd5a506df8511b52e55c7adc6d2e20a1f39 100644
--- a/src/heap/heap.cc
+++ b/src/heap/heap.cc
@@ -1847,7 +1847,7 @@ void Heap::FreeDeadArrayBuffers(bool from_scavenge) {
   }
 
   // Do not call through the api as this code is triggered while doing a GC.
-  amount_of_external_allocated_memory_ += FreeDeadArrayBuffersHelper(
+  amount_of_external_allocated_memory_ -= FreeDeadArrayBuffersHelper(
       isolate_,
       from_scavenge ? live_array_buffers_for_scavenge_ : live_array_buffers_,
       from_scavenge ? not_yet_discovered_array_buffers_for_scavenge_