Add flag to trace the amount of external memory.

src/heap-inl.h

Index: src/heap-inl.h
diff --git a/src/heap-inl.h b/src/heap-inl.h
index d5b0f50ea449e54f9afb24cda5b6ff20340805c1..4a827fef176fd57cafb2c929bb834cd229737aaa 100644
--- a/src/heap-inl.h
+++ b/src/heap-inl.h
@@ -487,6 +487,13 @@ intptr_t Heap::AdjustAmountOfExternalAllocatedMemory(
       amount_of_external_allocated_memory_at_last_global_gc_ = 0;
     }
   }
+  if (FLAG_trace_external_memory) {
+    PrintPID("%8.0f ms: ", isolate()->time_millis_since_init());
+    PrintF("Adjust amount of external memory: delta=%6" V8_PTR_PREFIX "d KB, "
+           " amount=%6" V8_PTR_PREFIX "d KB, isolate=0x%08" V8PRIxPTR ".\n",
+           change_in_bytes / 1024, amount_of_external_allocated_memory_ / 1024,
+           reinterpret_cast<intptr_t>(isolate()));
+  }
   ASSERT(amount_of_external_allocated_memory_ >= 0);
   return amount_of_external_allocated_memory_;
 }