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

Source/core/dom/Element.idl

Index: Source/core/dom/Element.idl
diff --git a/Source/core/dom/Element.idl b/Source/core/dom/Element.idl
index 2d0574cfa815845e8441d21b7fbdeb810a129890..14b102b1514a6535689804578da035cc8867778c 100644
--- a/Source/core/dom/Element.idl
+++ b/Source/core/dom/Element.idl
@@ -18,6 +18,9 @@
  * Boston, MA 02110-1301, USA.
  */
 
+// See https://goo.gl/1ipTpP for details.
+enum NativeScrollBehavior { "disable-native-scroll", "perform-before-native-scroll", "perform-after-native-scroll" };
+
 // https://dom.spec.whatwg.org/#interface-element
 
 [
@@ -103,6 +106,10 @@
     [MeasureAs=ElementOffsetWidth] readonly attribute long offsetWidth;
     [MeasureAs=ElementOffsetHeight] readonly attribute long offsetHeight;
 
+    // Scroll Customization API. See crbug.com/410974 for details.
+    [RuntimeEnabled=ScrollCustomization] void setApplyScroll(ScrollStateCallback scrollStateCallback, NativeScrollBehavior nativeScrollBehavior);
+    [RuntimeEnabled=ScrollCustomization] void setDistributeScroll(ScrollStateCallback scrollStateCallback, NativeScrollBehavior nativeScrollBehavior);
+
     // Non-standard APIs
     // https://www.w3.org/Bugs/Public/show_bug.cgi?id=19962
     [RaisesException, CustomElementCallbacks, MeasureAs=InsertAdjacentElement] Element insertAdjacentElement(DOMString where, Element element);