Fix slimming paint clipping and add a debug fill color

Source/core/rendering/compositing/CompositedLayerMapping.cpp

 /*
  * Copyright (C) 2009, 2010, 2011 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 2116 matching lines @@
 #if ENABLE(ASSERT)
     paintInfo.renderLayer->renderer()->assertSubtreeIsLaidOut();
 #endif
 
     if (paintInfo.renderLayer->compositingState() != PaintsIntoGroupedBacking) {
         // FIXME: GraphicsLayers need a way to split for RenderRegions.
         LayerPaintingInfo paintingInfo(paintInfo.renderLayer, dirtyRect, PaintBehaviorNormal, paintInfo.renderLayer->subpixelAccumulation());
         LayerPainter(*paintInfo.renderLayer).paintLayerContents(context, paintingInfo, paintLayerFlags);
 
         if (RuntimeEnabledFeatures::slimmingPaintEnabled()) {
+
+#ifndef NDEBUG
+            context->fillRect(dirtyRect, Color(0xFF, 0, 0));
+#endif
+
             if (RenderView* view = paintInfo.renderLayer->renderer()->view()) {
                 const PaintCommandList& paintCommandList = view->viewDisplayList().paintCommandList();
                 for (PaintCommandList::const_iterator it = paintCommandList.begin(); it != paintCommandList.end(); ++it)
                     context->drawDisplayList(it->get()->displayList.get());
             }
         }
 
         if (paintInfo.renderLayer->containsDirtyOverlayScrollbars())
             LayerPainter(*paintInfo.renderLayer).paintLayerContents(context, paintingInfo, paintLayerFlags | PaintLayerPaintingOverlayScrollbars);
     } else {
@@ skipping 222 matching lines @@
     } else if (graphicsLayer == m_scrollingBlockSelectionLayer.get()) {
         name = "Scrolling Block Selection Layer";
     } else {
         ASSERT_NOT_REACHED();
     }
 
     return name;
 }
 
 } // namespace blink