Use atomic operation to read the length of a fixed array.

src/factory.cc

Index: src/factory.cc
diff --git a/src/factory.cc b/src/factory.cc
index 3b3d0d29eadf6ccfd79555e5aa0e1ca269749533..7fc09867a554f501493d91bdb17b3b259444f891 100644
--- a/src/factory.cc
+++ b/src/factory.cc
@@ -1920,7 +1920,7 @@ void Factory::ReinitializeJSProxy(Handle<JSProxy> proxy, InstanceType type,
   if (size_difference > 0) {
     Address address = proxy->address();
     heap->CreateFillerObjectAt(address + map->instance_size(), size_difference);
-    heap->AdjustLiveBytes(address, -size_difference, Heap::FROM_MUTATOR);
+    heap->AdjustLiveBytes(address, -size_difference, Heap::SWEEPING_STARTED);
   }
 
   // Reset the map for the object.