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..a3d33fc7d5c4ce45f0751e52f75b8e6b03ccf950 100644
--- a/Source/core/dom/Element.idl
+++ b/Source/core/dom/Element.idl
@@ -20,6 +20,8 @@
 
 // https://dom.spec.whatwg.org/#interface-element
 
+callback ScrollStateFunctionCallback = void(ScrollState scrollState);
+
 [
     TypeChecking=Interface,
 ] interface Element : Node {
@@ -103,6 +105,14 @@
     [MeasureAs=ElementOffsetWidth] readonly attribute long offsetWidth;
     [MeasureAs=ElementOffsetHeight] readonly attribute long offsetHeight;
 
+    // Scroll Customization API. See crbug.com/410974 for details.
+  //    [RuntimeEnabled=ScrollCustomization] ScrollStateFunctionCallback getApplyScroll();
+  //    [RuntimeEnabled=ScrollCustomization] ScrollStateFunctionCallback getDistributeScroll();
+    [RuntimeEnabled=ScrollCustomization, CallWith=ScriptState] readonly attribute ScrollStateFunctionCallback applyScroll;
+    [RuntimeEnabled=ScrollCustomization, CallWith=ScriptState] readonly attribute ScrollStateFunctionCallback distributeScroll;
+    [RuntimeEnabled=ScrollCustomization] void setApplyScroll(ScrollStateCallback scrollStateCallback);
+    [RuntimeEnabled=ScrollCustomization] void setDistributeScroll(ScrollStateCallback scrollStateCallback);
+
     // Non-standard APIs
     // https://www.w3.org/Bugs/Public/show_bug.cgi?id=19962
     [RaisesException, CustomElementCallbacks, MeasureAs=InsertAdjacentElement] Element insertAdjacentElement(DOMString where, Element element);