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 a7827897eefc117f489b168451394a5a30d4e4ab..b1f28be93c8204cb9061eb0c2777c6f0d7edb796 100644
--- a/Source/core/rendering/RenderLayer.h
+++ b/Source/core/rendering/RenderLayer.h
@@ -474,12 +474,21 @@ 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;
 
@@ -492,7 +501,8 @@ public:
 
     bool paintsWithTransparency(PaintBehavior paintBehavior) const
     {
-        return isTransparent() && ((paintBehavior & PaintBehaviorFlattenCompositingLayers) || !isComposited());
+        // FIXME: shouldn't we return true if this layer has a backing but paints into its ancestor?

[Ian Vollick, 2013/10/01 19:46:22]

This should return true everywhere opacity won't be handled by the render
surface. I think you're right; if we're painting into our ancestor's backing and
we're transparent, the painting code has to handle the blending.

[shawnsingh, 2013/10/03 03:04:04]

done.

+        return isTransparent() && ((paintBehavior & PaintBehaviorFlattenCompositingLayers) || !backing());
     }
 
     bool paintsWithTransform(PaintBehavior) const;