Optimize ClipRects in the PrePaintTreeWalk

third_party/WebKit/Source/core/paint/PaintLayerPainter.cpp

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "core/paint/PaintLayerPainter.h"
 
 #include "core/frame/LocalFrame.h"
 #include "core/layout/LayoutView.h"
 #include "core/paint/ClipPathClipper.h"
 #include "core/paint/FilterPainter.h"
@@ skipping 136 matching lines @@
   DCHECK(m_paintLayer.isSelfPaintingLayer() ||
          m_paintLayer.hasSelfPaintingLayerDescendant());
 
   PaintLayerFlags localPaintFlags = paintFlags & ~(PaintLayerAppliedTransform);
   localPaintFlags |= PaintLayerPaintingCompositingAllPhases;
   return paintLayerContents(context, paintingInfo, localPaintFlags,
                             fragmentPolicy);
 }
 
 static bool shouldCreateSubsequence(const PaintLayer& paintLayer,
-                                    GraphicsContext& context,
+                                    const GraphicsContext& context,
                                     const PaintLayerPaintingInfo& paintingInfo,
                                     PaintLayerFlags paintFlags) {
   // Caching is not needed during printing.
   if (context.printing())
     return false;
 
   // Don't create subsequence for a composited layer because if it can be
   // cached, we can skip the whole painting in GraphicsLayer::paint() with
   // CachedDisplayItemList.  This also avoids conflict of
   // PaintLayer::previousXXX() when paintLayer is composited scrolling and is
@@ skipping 1030 matching lines @@
           context, layoutObject, PaintPhaseClippingMask))
     return;
 
   IntRect snappedClipRect = pixelSnappedIntRect(clipRect.rect());
   LayoutObjectDrawingRecorder drawingRecorder(
       context, layoutObject, PaintPhaseClippingMask, snappedClipRect);
   context.fillRect(snappedClipRect, Color::black);
 }
 
 }  // namespace blink