commit 7a8497c0df722b1ed145b99a82c71ed1f7b1d6ce
Author: Markus Stange <mstange@themasta.com>
Date:   Thu Oct 9 21:26:27 2014 -0400

    Bug 1074280 - Use AsContainerLayer() in order to avoid a bad cast. r=roc, a=bkerensa

	Modified   gfx/layers/basic/BasicLayerManager.cpp
diff --git a/gfx/layers/basic/BasicLayerManager.cpp b/gfx/layers/basic/BasicLayerManager.cpp
index 5a3a1f6..ff42bc0 100644
--- a/gfx/layers/basic/BasicLayerManager.cpp
+++ b/gfx/layers/basic/BasicLayerManager.cpp
@@ -901,18 +901,17 @@ BasicLayerManager::PaintLayer(gfxContext* aTarget,
   RenderTraceScope trace("BasicLayerManager::PaintLayer", "707070");
 
   const nsIntRect* clipRect = aLayer->GetEffectiveClipRect();
-  // aLayer might not be a container layer, but if so we take care not to use
-  // the container variable
-  BasicContainerLayer* container = static_cast<BasicContainerLayer*>(aLayer);
-  bool needsGroup = aLayer->GetFirstChild() &&
+  BasicContainerLayer* container =
+    static_cast<BasicContainerLayer*>(aLayer->AsContainerLayer());
+  bool needsGroup = container &&
                     container->UseIntermediateSurface();
   BasicImplData* data = ToData(aLayer);
   bool needsClipToVisibleRegion =
     data->GetClipToVisibleRegion() && !aLayer->AsThebesLayer();
-  NS_ASSERTION(needsGroup || !aLayer->GetFirstChild() ||
+  NS_ASSERTION(needsGroup || !container ||
                container->GetOperator() == CompositionOp::OP_OVER,
                "non-OVER operator should have forced UseIntermediateSurface");
-  NS_ASSERTION(!aLayer->GetFirstChild() || !aLayer->GetMaskLayer() ||
+  NS_ASSERTION(!container || !aLayer->GetMaskLayer() ||
                container->UseIntermediateSurface(),
                "ContainerLayer with mask layer should force UseIntermediateSurface");