Update scroll parent correctly for squashing layers.

Source/platform/graphics/CompositingReasons.h

 // 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.
 
 #ifndef CompositingReasons_h
 #define CompositingReasons_h
 
 #include "wtf/MathExtras.h"
 #include <stdint.h>
 
@@ skipping 86 matching lines @@
     | CompositingReasonTransformWithCompositedDescendants
     | CompositingReasonOpacityWithCompositedDescendants
     | CompositingReasonMaskWithCompositedDescendants
     | CompositingReasonFilterWithCompositedDescendants
     | CompositingReasonBlendingWithCompositedDescendants
     | CompositingReasonIsolateCompositedDescendants
     | CompositingReasonPreserve3DWith3DDescendants; // preserve-3d has to create backing store to ensure that 3d-transformed elements intersect.
 
 const uint64_t CompositingReasonComboSquashableReasons =
     CompositingReasonOverlap
-    | CompositingReasonAssumedOverlap;
+    | CompositingReasonAssumedOverlap
+    | CompositingReasonOverflowScrollingParent;
 
 typedef uint64_t CompositingReasons;
 
 struct CompositingReasonStringMap {
     CompositingReasons reason;
     const char* shortName;
     const char* description;
 };
 
 static const CompositingReasonStringMap compositingReasonStringMap[] = {
@@ skipping 136 matching lines @@
         "layerForMask",
         "Secondary layer, to contain the mask contents" },
     { CompositingReasonLayerForClippingMask,
         "layerForClippingMask",
         "Secondary layer, for clipping mask" }
 };
 
 } // namespace WebCore
 
 #endif // CompositingReasons_h