Bug 811831 - Don't create layers for nsDisplayOpacity items that have an opacity of 0. r=roc

layout/base/nsDisplayList.cpp

@@ -2756,6 +2756,9 @@ already_AddRefed<Layer>
 nsDisplayOpacity::BuildLayer(nsDisplayListBuilder* aBuilder,
                              LayerManager* aManager,
                              const ContainerParameters& aContainerParameters) {
+  if (mFrame->GetStyleDisplay()->mOpacity == 0) {
+    return nullptr;
+  }
   nsRefPtr<Layer> container = aManager->GetLayerBuilder()->
     BuildContainerLayerFor(aBuilder, aManager, mFrame, this, mList,
                            aContainerParameters, nullptr);