Add comment for why lock is added.

AsyncDisplayKit/ASCellNode.mm

@@ -126,6 +126,8 @@
   CGSize oldSize = self.calculatedSize;
   [super __setNeedsLayout];
   
+  //Adding this lock because lock used to be held when this method was called. Not sure if it's necessary for
+  //didRelayoutFromOldSize:toNewSize:
   ASDN::MutexLocker l(_propertyLock);
   [self didRelayoutFromOldSize:oldSize toNewSize:self.calculatedSize];
 }