Lower limit for external allocation and improve tracing.

src/heap-inl.h

Index: src/heap-inl.h
diff --git a/src/heap-inl.h b/src/heap-inl.h
index eb9d9c09d37f79c57a88fc8598a7172bfc4c8bf0..2d0968d34195d7ab6a2f1c87f2518323facf55b4 100644
--- a/src/heap-inl.h
+++ b/src/heap-inl.h
@@ -550,7 +550,7 @@ intptr_t Heap::AdjustAmountOfExternalAllocatedMemory(
     if (amount >= 0) {
       amount_of_external_allocated_memory_ = amount;
     } else {
-      // Give up and reset the counters in case of an overflow.
+      // Give up and reset the counters in case of an underflow.
       amount_of_external_allocated_memory_ = 0;
       amount_of_external_allocated_memory_at_last_global_gc_ = 0;
     }
@@ -558,8 +558,11 @@ intptr_t Heap::AdjustAmountOfExternalAllocatedMemory(
   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,
+           "amount=%6" V8_PTR_PREFIX "d KB, since_gc=%6" V8_PTR_PREFIX "d KB, "
+           "isolate=0x%08" V8PRIxPTR ".\n",
+           change_in_bytes / KB,
+           amount_of_external_allocated_memory_ / KB,
+           PromotedExternalMemorySize() / KB,
            reinterpret_cast<intptr_t>(isolate()));
   }
   ASSERT(amount_of_external_allocated_memory_ >= 0);