[RLS] Don't create PaintLayerCompositor layers.

third_party/WebKit/Source/core/layout/compositing/PaintLayerCompositor.h

 /*
  * Copyright (C) 2009 Apple Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 105 matching lines @@
   GraphicsLayer* fixedRootBackgroundLayer() const;
   void setNeedsUpdateFixedBackground() { m_needsUpdateFixedBackground = true; }
 
   // Issue paint invalidations of the appropriate layers when the given Layer
   // starts or stops being composited.
   void paintInvalidationOnCompositingChange(PaintLayer*);
 
   void fullyInvalidatePaint();
 
   PaintLayer* rootLayer() const;
+
+  GraphicsLayer* containerLayer() const { return m_containerLayer.get(); }
+  GraphicsLayer* frameScrollLayer() const { return m_scrollLayer.get(); }
+  GraphicsLayer* rootContentLayer() const { return m_rootContentLayer.get(); }
+  GraphicsLayer* layerForHorizontalScrollbar() const {
+    return m_layerForHorizontalScrollbar.get();
+  }
+  GraphicsLayer* layerForVerticalScrollbar() const {
+    return m_layerForVerticalScrollbar.get();
+  }
+  GraphicsLayer* layerForScrollCorner() const {
+    return m_layerForScrollCorner.get();
+  }
+
+  // In root layer scrolling mode, returns the LayoutView's main GraphicsLayer.
+  // In non-RLS mode, returns the outermost PaintLayerCompositor layer.
   GraphicsLayer* rootGraphicsLayer() const;
-  GraphicsLayer* frameScrollLayer() const;
+
+  // In root layer scrolling mode, this is the LayoutView's scroll layer.
+  // In non-RLS mode, this is the same as frameScrollLayer().
   GraphicsLayer* scrollLayer() const;
-  GraphicsLayer* containerLayer() const;
-  GraphicsLayer* rootContentLayer() const;
 
   enum RootLayerAttachment {
     RootLayerUnattached,
     RootLayerAttachedViaChromeClient,
     RootLayerAttachedViaEnclosingFrame
   };
 
   RootLayerAttachment getRootLayerAttachment() const {
     return m_rootLayerAttachment;
   }
@@ skipping 17 matching lines @@
   void frameViewDidChangeLocation(const IntPoint& contentsOffset);
   void frameViewDidChangeSize();
   void frameViewDidScroll();
   void frameViewScrollbarsExistenceDidChange();
   void rootFixedBackgroundsChanged();
 
   bool scrollingLayerDidChange(PaintLayer*);
 
   std::unique_ptr<JSONObject> layerTreeAsJSON(LayerTreeFlags) const;
 
-  GraphicsLayer* layerForHorizontalScrollbar() const {
-    return m_layerForHorizontalScrollbar.get();
-  }
-  GraphicsLayer* layerForVerticalScrollbar() const {
-    return m_layerForVerticalScrollbar.get();
-  }
-  GraphicsLayer* layerForScrollCorner() const {
-    return m_layerForScrollCorner.get();
-  }
-
   void setTracksRasterInvalidations(bool);
 
   String debugName(const GraphicsLayer*) const override;
   DocumentLifecycle& lifecycle() const;
 
   void updatePotentialCompositingReasonsFromStyle(PaintLayer*);
 
   // Whether the layer could ever be composited.
   bool canBeComposited(const PaintLayer*) const;
 
@@ skipping 52 matching lines @@
 
   void updateContainerSizes();
 
   // Checks the given graphics layer against the compositor's horizontal and
   // vertical scrollbar graphics layers, returning the associated Scrollbar
   // instance if any, else nullptr.
   Scrollbar* graphicsLayerToScrollbar(const GraphicsLayer*) const;
 
   bool isMainFrame() const;
   VisualViewport& visualViewport() const;
+  GraphicsLayer* parentForContentLayers() const;
 
   LayoutView& m_layoutView;
-  std::unique_ptr<GraphicsLayer> m_rootContentLayer;
 
   CompositingReasonFinder m_compositingReasonFinder;
 
   CompositingUpdateType m_pendingUpdateType;
 
   bool m_hasAcceleratedCompositing;
   bool m_compositing;
 
   // The root layer doesn't composite if it's a non-scrollable frame.
   // So, after a layout we set this dirty bit to know that we need
   // to recompute whether the root layer should composite even if
   // none of its descendants composite.
   // FIXME: Get rid of all the callers of setCompositingModeEnabled
   // except the one in updateIfNeeded, then rename this to
   // m_compositingDirty.
   bool m_rootShouldAlwaysCompositeDirty;
   bool m_needsUpdateFixedBackground;
   bool m_isTrackingRasterInvalidations;  // Used for testing.
   bool m_inOverlayFullscreenVideo;
 
   RootLayerAttachment m_rootLayerAttachment;
   bool m_pendingChromeClientAttachment;
 
-  // Enclosing container layer, which clips for iframe content
+  // Outermost layer, holds overflow controls and the container layer
+  std::unique_ptr<GraphicsLayer> m_overflowControlsHostLayer;
+
+  // Clips for iframe content
   std::unique_ptr<GraphicsLayer> m_containerLayer;
+
+  // Scrolls with the FrameView
   std::unique_ptr<GraphicsLayer> m_scrollLayer;
 
-  // Enclosing layer for overflow controls and the clipping layer
-  std::unique_ptr<GraphicsLayer> m_overflowControlsHostLayer;
+  // Innermost layer, parent of LayoutView main GraphicsLayer
+  std::unique_ptr<GraphicsLayer> m_rootContentLayer;
 
   // Layers for overflow controls
   std::unique_ptr<GraphicsLayer> m_layerForHorizontalScrollbar;
   std::unique_ptr<GraphicsLayer> m_layerForVerticalScrollbar;
   std::unique_ptr<GraphicsLayer> m_layerForScrollCorner;
 };
 
 }  // namespace blink
 
 #endif  // PaintLayerCompositor_h