[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 db3753a8a98516b8d6594e06ce4365272f5b8981..309d65a956b62f4db6ede4292f23e6b6820e0116 100644
--- a/Source/core/rendering/RenderLayer.h
+++ b/Source/core/rendering/RenderLayer.h
@@ -589,6 +589,8 @@ private:
 
     bool isDirtyStackingContainer() const { return m_zOrderListsDirty && isStackingContainer(); }
 
+    bool hasNonAcceleratedBlendedChildInCurrentStackingContext() const;
+
     void setAncestorChainHasSelfPaintingLayerDescendant();
     void dirtyAncestorChainHasSelfPaintingLayerDescendantStatus();
 
@@ -895,6 +897,8 @@ 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_isolateForDescendantBlendMode : 1; // A layer with mix-blend-mode should only be blended
+        // with content underneath this layer, within the same stacking context.
     unsigned m_paintingInsideReflection : 1; // A state bit tracking if we are painting inside a replica.
     unsigned m_repaintStatus : 2; // RepaintStatus