Restore the "don't flatten scrolling content layer" logic.

Source/core/layout/compositing/CompositedDeprecatedPaintLayerMapping.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 1453 matching lines @@
     ApplyToGraphicsLayersMode mode = ApplyToLayersAffectedByPreserve3D;
     ApplyToGraphicsLayers(this, functor, mode);
 
     // Note, if we apply perspective, we have to set should flatten differently
     // so that the transform propagates to child layers correctly.
     if (hasChildTransformLayer()) {
         ApplyToGraphicsLayers(this, [](GraphicsLayer* layer) {
             layer->setShouldFlattenTransform(false);
         }, ApplyToChildContainingLayers);
     }
+
+    // Regardless, mark the scrolling contents layer and scrolling block
+    // selection layer (if they exist) as not flattening. Having them flatten
+    // causes unclipped render surfaces which cause bugs.
+    // http://crbug.com/521768
+    if (hasScrollingLayer()) {
+        m_scrollingContentsLayer->setShouldFlattenTransform(false);
+        if (m_scrollingBlockSelectionLayer)
+            m_scrollingBlockSelectionLayer->setShouldFlattenTransform(false);
+    }
 }
 
 bool CompositedDeprecatedPaintLayerMapping::updateForegroundLayer(bool needsForegroundLayer)
 {
     bool layerChanged = false;
     if (needsForegroundLayer) {
         if (!m_foregroundLayer) {
             m_foregroundLayer = createGraphicsLayer(CompositingReasonLayerForForeground);
             m_foregroundLayer->setPaintingPhase(GraphicsLayerPaintForeground);
             layerChanged = true;
@@ skipping 891 matching lines @@
     } else if (graphicsLayer == m_scrollingBlockSelectionLayer.get()) {
         name = "Scrolling Block Selection Layer";
     } else {
         ASSERT_NOT_REACHED();
     }
 
     return name;
 }
 
 } // namespace blink