Expose scroll customization for touch to JS (behind REF).

Source/core/page/scrolling/ScrollState.idl

 // Copyright 2015 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.
 
-// http://dev.w3.org/houdini/css-scroll-api/
+// https://docs.google.com/document/d/1VnvAqeWFG9JFZfgG5evBqrLGDZYRE5w6G5jEDORekPY
 
 [
     Constructor(optional double deltaX = 0,
                 optional double deltaY = 0,
                 optional double deltaGranularity = 0,
                 optional double velocityX = 0,
                 optional double velocityY = 0,
                 optional boolean inInertialPhase = false,
                 optional boolean isBeginning = false,
                 optional boolean isEnding = false),
     WillBeGarbageCollected,
     RuntimeEnabled=ScrollCustomization
 ] interface ScrollState {
     readonly attribute double deltaX;
     readonly attribute double deltaY;
     readonly attribute double deltaGranularity;
     readonly attribute double velocityX;
     readonly attribute double velocityY;
     readonly attribute boolean inInertialPhase;
     readonly attribute boolean isEnding;
     readonly attribute boolean isBeginning;
     readonly attribute boolean fromUserInput;
     readonly attribute boolean shouldPropagate;
 
     [RaisesException] void consumeDelta(double x, double y);
-    // FIXME: implement distributeToScrollChainDescendent.
+    void distributeToScrollChainDescendant();
 };