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

Source/core/layout/compositing/CompositedDeprecatedPaintLayerMapping.cpp

Index: Source/core/layout/compositing/CompositedDeprecatedPaintLayerMapping.cpp
diff --git a/Source/core/layout/compositing/CompositedDeprecatedPaintLayerMapping.cpp b/Source/core/layout/compositing/CompositedDeprecatedPaintLayerMapping.cpp
index ff9151aac9a36797b0029dedc8093deb200bf711..3a2c3683122e2867aee88aab8fefe3fec6ada5e3 100644
--- a/Source/core/layout/compositing/CompositedDeprecatedPaintLayerMapping.cpp
+++ b/Source/core/layout/compositing/CompositedDeprecatedPaintLayerMapping.cpp
@@ -1471,6 +1471,16 @@ void CompositedDeprecatedPaintLayerMapping::updateShouldFlattenTransform()
             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)