[heap] Properly decrement amount of externally allocated memory

TBR=mstarzinger@chromium.org
BUG=chromium:526244
LOG=N

Review URL: https://codereview.chromium.org/1329493002 .

Cr-Commit-Position: refs/heads/master@{#30487}

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_