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..64db89ab6466d84fe6483107752159d68f8dbe0d 100644
--- a/Source/core/dom/Element.idl
+++ b/Source/core/dom/Element.idl
@@ -18,6 +18,11 @@
  * Boston, MA 02110-1301, USA.
  */
 
+// See
+// https://docs.google.com/document/d/1VnvAqeWFG9JFZfgG5evBqrLGDZYRE5w6G5jEDORekPY
+// for details.
+enum NativeScrollBehavior { "disable-native-scroll", "perform-before-native-scroll", "perform-after-native-scroll" };
+
 // https://dom.spec.whatwg.org/#interface-element
 
 [
@@ -103,6 +108,10 @@
     [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);

[skobes, 2015/07/15 19:52:40]

You mentioned the bindings are temporary... does that mean the eventual plan is
to allow direct assignment to element.applyScroll?  IMHO that's more intuitive
than setApplyScroll, and we shouldn't compromise the API for ease of
implementation.

[tdresser, 2015/07/16 17:11:14]

The final API is still somewhat up in the air, hence the temporary bindings. The
plan is to align this API with the rest of the houdini effort (custom layout and
custom paint).

In particular, see http://dev.w3.org/houdini/css-paint-api/#issue-5a80a930. We
may end up with a registerCustomScroller method similar to registerCustomPaint,
and we may pass in a class with applyScroll and distributeScroll methods. 

One option is to have a class available, perhaps CustomScroller. You'd inherit
from CustomScroller, and be able to call super to access the default scrolling
implementation.

There's still some work to be done to finalize the custom layout and custom
paint API's though, so for now we're just going to land this temporary API so
folks can get a feel for using scroll customization.

[skobes, 2015/07/16 18:56:40]

Sounds good, thanks for adding context here.

+    [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);