Refactor root element background painting

Source/core/paint/BoxPainter.h

Index: Source/core/paint/BoxPainter.h
diff --git a/Source/core/paint/BoxPainter.h b/Source/core/paint/BoxPainter.h
index e2a1cbc96c5bde3e39c95c0e21ed2e53f80d3067..8ccfa4d80de0373dc9ea9a10ff4c74cf5e83857a 100644
--- a/Source/core/paint/BoxPainter.h
+++ b/Source/core/paint/BoxPainter.h
@@ -25,11 +25,18 @@ public:
     void paintBoxDecorationBackground(const PaintInfo&, const LayoutPoint&);
     void paintMask(const PaintInfo&, const LayoutPoint&);
     void paintClippingMask(const PaintInfo&, const LayoutPoint&);
-    void paintFillLayers(const PaintInfo&, const Color&, const FillLayer&, const LayoutRect&, BackgroundBleedAvoidance = BackgroundBleedNone, SkXfermode::Mode = SkXfermode::kSrcOver_Mode, LayoutObject* backgroundObject = 0);
+
+    typedef Vector<const FillLayer*, 8> FillLayerOcclusionOutputList;
+    // Returns true if the result fill layers have non-associative blending or compositing mode.
+    // (i.e. The rendering will be different without creating isolation group by context.saveLayer().)
+    // Note that the output list will be in top-bottom order.
+    bool calculateFillLayerOcclusionCulling(FillLayerOcclusionOutputList &reversedPaintList, const FillLayer&);
+
+    void paintFillLayers(const PaintInfo&, const Color&, const FillLayer&, const LayoutRect&, BackgroundBleedAvoidance = BackgroundBleedNone, SkXfermode::Mode = SkXfermode::kSrcOver_Mode, LayoutObject* backgroundObject = nullptr);
     void paintMaskImages(const PaintInfo&, const LayoutRect&);
     void paintBoxDecorationBackgroundWithRect(const PaintInfo&, const LayoutPoint&, const LayoutRect&);
-    static void paintFillLayerExtended(LayoutBoxModelObject&, const PaintInfo&, const Color&, const FillLayer&, const LayoutRect&, BackgroundBleedAvoidance, InlineFlowBox* = 0, const LayoutSize& = LayoutSize(), SkXfermode::Mode = SkXfermode::kSrcOver_Mode, LayoutObject* backgroundObject = 0, bool skipBaseColor = false);
-    static void calculateBackgroundImageGeometry(LayoutBoxModelObject&, const LayoutBoxModelObject* paintContainer, const FillLayer&, const LayoutRect& paintRect, BackgroundImageGeometry&, LayoutObject* = 0);
+    static void paintFillLayerExtended(LayoutBoxModelObject&, const PaintInfo&, const Color&, const FillLayer&, const LayoutRect&, BackgroundBleedAvoidance, InlineFlowBox* = nullptr, const LayoutSize& = LayoutSize(), SkXfermode::Mode = SkXfermode::kSrcOver_Mode, LayoutObject* backgroundObject = nullptr);
+    static void calculateBackgroundImageGeometry(LayoutBoxModelObject&, const LayoutBoxModelObject* paintContainer, const FillLayer&, const LayoutRect& paintRect, BackgroundImageGeometry&, LayoutObject* backgroundObject = nullptr);
     static InterpolationQuality chooseInterpolationQuality(LayoutObject&, GraphicsContext*, Image*, const void*, const LayoutSize&);
     static bool paintNinePieceImage(LayoutBoxModelObject&, GraphicsContext*, const LayoutRect&, const ComputedStyle&, const NinePieceImage&, SkXfermode::Mode = SkXfermode::kSrcOver_Mode);
     static void paintBorder(LayoutBoxModelObject&, const PaintInfo&, const LayoutRect&, const ComputedStyle&, BackgroundBleedAvoidance = BackgroundBleedNone, bool includeLogicalLeftEdge = true, bool includeLogicalRightEdge = true);
@@ -39,19 +46,15 @@ public:
 
 private:
     void paintBackground(const PaintInfo&, const LayoutRect&, const Color& backgroundColor, BackgroundBleedAvoidance = BackgroundBleedNone);
-    void paintRootBoxFillLayers(const PaintInfo&);
-    void paintFillLayer(const PaintInfo&, const Color&, const FillLayer&, const LayoutRect&, BackgroundBleedAvoidance, SkXfermode::Mode, LayoutObject* backgroundObject, bool skipBaseColor = false);
-    static void paintRootBackgroundColor(LayoutObject&, const PaintInfo&, const LayoutRect&, const Color&);
+    void paintFillLayer(const PaintInfo&, const Color&, const FillLayer&, const LayoutRect&, BackgroundBleedAvoidance, SkXfermode::Mode, LayoutObject* backgroundObject = nullptr);
     static FloatRoundedRect backgroundRoundedRectAdjustedForBleedAvoidance(LayoutObject&, const LayoutRect&, BackgroundBleedAvoidance, InlineFlowBox*, const LayoutSize&, bool includeLogicalLeftEdge, bool includeLogicalRightEdge);
     static FloatRoundedRect getBackgroundRoundedRect(LayoutObject&, const LayoutRect&, InlineFlowBox*, LayoutUnit inlineBoxWidth, LayoutUnit inlineBoxHeight,
         bool includeLogicalLeftEdge, bool includeLogicalRightEdge);
-    static bool isDocumentElementWithOpaqueBackground(LayoutObject&);
     static void applyBoxShadowForBackground(GraphicsContext*, LayoutObject&);
     static bool fixedBackgroundPaintsInLocalCoordinates(const LayoutObject&);
     static IntSize calculateFillTileSize(const LayoutBoxModelObject&, const FillLayer&, const IntSize& scaledPositioningAreaSize);
 
     LayoutRect boundsForDrawingRecorder(const LayoutPoint& paintOffset);
-    LayoutRect rootBackgroundRect();
 
     // FIXME: this should be const.
     LayoutBox& m_layoutBox;