Make compositingState explicit

Source/core/rendering/RenderLayer.h

Index: Source/core/rendering/RenderLayer.h
diff --git a/Source/core/rendering/RenderLayer.h b/Source/core/rendering/RenderLayer.h
index 2b071c9beae4337f63d2b430a4a18b5f53f10a31..3ea2dc7622c0b0533e520c0881e00529559b71e6 100644
--- a/Source/core/rendering/RenderLayer.h
+++ b/Source/core/rendering/RenderLayer.h
@@ -462,12 +462,21 @@ public:
     // Only safe to call from RenderLayerModelObject::destroyLayer()
     void operator delete(void*);
 
-    bool isComposited() const { return m_backing != 0; }
-    bool hasCompositedMask() const;
-    bool hasCompositedClippingMask() const;
-    RenderLayerBacking* backing() const { return m_backing.get(); }
+    CompositingState compositingState() const;
+
+    RenderLayerBacking* backing() const
+    {
+        // NOTE: Checking whether backing exists is equivalent to asking if you are in one of these two compositing states.
+        //       If this is why you are using backing(), please try to use compositingState() instead.
+        ASSERT(compositingState() == HasOwnBackingButPaintsIntoAncestor || compositingState() == PaintsIntoOwnBacking);
+        return m_backing.get();
+    }
+
     RenderLayerBacking* ensureBacking();
     void clearBacking(bool layerBeingDestroyed = false);
+
+    bool hasCompositedMask() const;
+    bool hasCompositedClippingMask() const;
     bool needsCompositedScrolling() const;
     bool needsToBeStackingContainer() const;
 
@@ -480,7 +489,7 @@ public:
 
     bool paintsWithTransparency(PaintBehavior paintBehavior) const
     {
-        return isTransparent() && ((paintBehavior & PaintBehaviorFlattenCompositingLayers) || !isComposited());
+        return isTransparent() && ((paintBehavior & PaintBehaviorFlattenCompositingLayers) || compositingState() != PaintsIntoOwnBacking);
     }
 
     bool paintsWithTransform(PaintBehavior) const;