Bug 1232042 - Addendum: Add comment for mLayerManager check. r=jrmuizel

gfx/layers/ipc/CompositorParent.cpp

@@ -875,6 +875,7 @@ CompositorParent::RecvFlushRendering()
 bool
 CompositorParent::RecvForcePresent()
 {
+  // During the shutdown sequence mLayerManager may be null
   if (mLayerManager) {
     mLayerManager->ForcePresent();
   }