[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 1c957f658880c3adee84cf105dbd4220ffa8f39d..5468daed672b4e28f335b98b6337b6b0e9f00f56 100644
--- a/Source/core/rendering/RenderLayer.h
+++ b/Source/core/rendering/RenderLayer.h
@@ -286,6 +286,8 @@ public:
     void setHasOutOfFlowPositionedDescendant(bool hasDescendant) { m_hasOutOfFlowPositionedDescendant = hasDescendant; }
     void setHasOutOfFlowPositionedDescendantDirty(bool dirty) { m_hasOutOfFlowPositionedDescendantDirty = dirty; }
 
+    bool childLayerHasBlendMode() const { ASSERT(!m_childLayerHasBlendModeStatusDirty); return m_childLayerHasBlendMode; }
+
     bool hasUnclippedDescendant() const { return m_hasUnclippedDescendant; }
     void setHasUnclippedDescendant(bool hasDescendant) { m_hasUnclippedDescendant = hasDescendant; }
     void updateHasUnclippedDescendant();
@@ -764,6 +766,9 @@ 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.
@@ -856,6 +861,9 @@ 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_visibleContentStatusDirty : 1;