Allow proper highlighting on universal overflow scroll.

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 52 matching lines @@
 const uint64_t CompositingReasonLayerForVerticalScrollbar              = UINT64_C(1) << 37;
 const uint64_t CompositingReasonLayerForScrollCorner                   = UINT64_C(1) << 38;
 const uint64_t CompositingReasonLayerForScrollingContents              = UINT64_C(1) << 39;
 const uint64_t CompositingReasonLayerForScrollingContainer             = UINT64_C(1) << 40;
 const uint64_t CompositingReasonLayerForSquashingContents              = UINT64_C(1) << 41;
 const uint64_t CompositingReasonLayerForSquashingContainer             = UINT64_C(1) << 42;
 const uint64_t CompositingReasonLayerForForeground                     = UINT64_C(1) << 43;
 const uint64_t CompositingReasonLayerForBackground                     = UINT64_C(1) << 44;
 const uint64_t CompositingReasonLayerForMask                           = UINT64_C(1) << 45;
 const uint64_t CompositingReasonLayerForClippingMask                   = UINT64_C(1) << 46;
+const uint64_t CompositingReasonLayerForScrollingBlockSelection        = UINT64_C(1) << 47;
 
 // Various combinations of compositing reasons are defined here also, for more intutive and faster bitwise logic.
 const uint64_t CompositingReasonComboAllDirectReasons =
     CompositingReason3DTransform
     | CompositingReasonVideo
     | CompositingReasonCanvas
     | CompositingReasonPlugin
     | CompositingReasonIFrame
     | CompositingReasonBackfaceVisibilityHidden
     | CompositingReasonActiveAnimation
@@ skipping 188 matching lines @@
         "layerForForeground",
         "Secondary layer, to contain any normal flow and positive z-index contents on top of a negative z-index layer" },
     { CompositingReasonLayerForBackground,
         "layerForBackground",
         "Secondary layer, to contain acceleratable background content" },
     { CompositingReasonLayerForMask,
         "layerForMask",
         "Secondary layer, to contain the mask contents" },
     { CompositingReasonLayerForClippingMask,
         "layerForClippingMask",
-        "Secondary layer, for clipping mask" }
+        "Secondary layer, for clipping mask" },
+    { CompositingReasonLayerForScrollingBlockSelection,
+        "layerForScrollingBlockSelection",
+        "Secondary layer, to house block selection gaps for composited scrolling with no scrolling contents" },
 };
 
 } // namespace WebCore
 
 #endif // CompositingReasons_h