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

Source/core/page/scrolling/ScrollStateCallback.idl

Index: Source/core/page/scrolling/ScrollStateCallback.idl
diff --git a/Source/core/page/scrolling/ScrollStateCallback.idl b/Source/core/page/scrolling/ScrollStateCallback.idl
new file mode 100644
index 0000000000000000000000000000000000000000..423710a97e97e0270be4d5466f001183f6e07b3e
--- /dev/null
+++ b/Source/core/page/scrolling/ScrollStateCallback.idl
@@ -0,0 +1,11 @@
+// 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.
+
+// https://docs.google.com/document/d/1VnvAqeWFG9JFZfgG5evBqrLGDZYRE5w6G5jEDORekPY
+
+[
+    RuntimeEnabled=ScrollCustomization
+] callback interface ScrollStateCallback {
+    void handleEvent(ScrollState scrollState);
+};