Add support for accelerated fixed root background

Source/core/rendering/RenderLayer.cpp

Index: Source/core/rendering/RenderLayer.cpp
diff --git a/Source/core/rendering/RenderLayer.cpp b/Source/core/rendering/RenderLayer.cpp
index a0e473550efdb477530b6934bc037f101901eafe..7baf7edc03eaaca5837c346f513afc3b1025e21a 100644
--- a/Source/core/rendering/RenderLayer.cpp
+++ b/Source/core/rendering/RenderLayer.cpp
@@ -3496,15 +3496,21 @@ static inline bool shouldSuppressPaintingLayer(RenderLayer* layer)
     return false;
 }
 
+static bool paintForFixedRootBackground(const RenderLayer* layer, RenderLayer::PaintLayerFlags paintFlags)
+{
+    return layer->renderer()->isRoot() && (paintFlags & RenderLayer::PaintLayerPaintingRootBackgroundOnly);
+}
+
 void RenderLayer::paintLayer(GraphicsContext* context, const LayerPaintingInfo& paintingInfo, PaintLayerFlags paintFlags)
 {
     if (isComposited()) {
         // The updatingControlTints() painting pass goes through compositing layers,
         // but we need to ensure that we don't cache clip rects computed with the wrong root in this case.
-        if (context->updatingControlTints() || (paintingInfo.paintBehavior & PaintBehaviorFlattenCompositingLayers))
+        if (context->updatingControlTints() || (paintingInfo.paintBehavior & PaintBehaviorFlattenCompositingLayers)) {
             paintFlags |= PaintLayerTemporaryClipRects;
-        else if (!backing()->paintsIntoCompositedAncestor()
-            && !shouldDoSoftwarePaint(this, paintFlags & PaintLayerPaintingReflection)) {
+        } else if (!backing()->paintsIntoCompositedAncestor()
+            && !shouldDoSoftwarePaint(this, paintFlags & PaintLayerPaintingReflection)
+            && !paintForFixedRootBackground(this, paintFlags)) {
             // If this RenderLayer should paint into its backing, that will be done via RenderLayerBacking::paintIntoLayer().
             return;
         }