Add wheel support to sky-scrollable

sky/engine/core/events/WheelEvent.idl

Index: sky/engine/core/events/WheelEvent.idl
diff --git a/sky/engine/core/events/WheelEvent.idl b/sky/engine/core/events/WheelEvent.idl
new file mode 100644
index 0000000000000000000000000000000000000000..e7c9adb09ea97118b1c4cc6f486176c9ffad2b26
--- /dev/null
+++ b/sky/engine/core/events/WheelEvent.idl
@@ -0,0 +1,12 @@
+// 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.
+
+[
+    EventConstructor,
+] interface WheelEvent : Event {
+    [InitializedByEventConstructor] readonly attribute double x;
+    [InitializedByEventConstructor] readonly attribute double y;
+    [InitializedByEventConstructor] readonly attribute double offsetX;
+    [InitializedByEventConstructor] readonly attribute double offsetY;
+};