[CSS Blending] Implement mix-blend-mode in software.

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 c1b2100b1bbdcf24a3b8824f803f4b852d10bd79..c7ea8831272888bdaaef3a320f192ddc9e6caee8 100644
--- a/Source/core/rendering/RenderLayer.h
+++ b/Source/core/rendering/RenderLayer.h
@@ -802,6 +802,9 @@ private:
     void dirtyAncestorChainVisibleDescendantStatus();
     void setAncestorChainHasVisibleDescendant();
 
+    void dirtyAncestorChainBlendedDescendantStatus();
+    void setAncestorChainBlendedDescendant();
+
     void updateDescendantDependentFlags();
 
     // This flag is computed by RenderLayerCompositor, which knows more about 3d hierarchies than we do.
@@ -900,6 +903,9 @@ protected:
     unsigned m_usedTransparency : 1; // Tracks whether we need to close a transparent layer, i.e., whether
                                  // we ended up painting this layer or any descendants (and therefore need to
                                  // blend).
+    unsigned m_childLayerHasBlendMode : 1;
+    unsigned m_childLayerHasBlendModeStatusDirty : 1;
+
     unsigned m_paintingInsideReflection : 1; // A state bit tracking if we are painting inside a replica.
     unsigned m_repaintStatus : 2; // RepaintStatus