Sync the ClipboardEvent interface with the spec

Source/core/events/ClipboardEvent.idl

Index: Source/core/events/ClipboardEvent.idl
diff --git a/Source/core/events/ClipboardEvent.idl b/Source/core/events/ClipboardEvent.idl
index 96db58949e66b63c5606f7bed33afa07703db775..8913e834420fd6a11b403e97503d00dff32da76f 100644
--- a/Source/core/events/ClipboardEvent.idl
+++ b/Source/core/events/ClipboardEvent.idl
@@ -2,7 +2,10 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// https://w3c.github.io/clipboard-apis/#clipboard-event-interfaces
+
+// TODO(philipj): There should be a constructor which takes a ClipboardEventInit
+// dictionary.
 interface ClipboardEvent : Event {
     readonly attribute DataTransfer clipboardData;
 };
-