Improve the declared IDL type for postMessage()'s transferables

Source/core/frame/Window.idl

Index: Source/core/frame/Window.idl
diff --git a/Source/core/frame/Window.idl b/Source/core/frame/Window.idl
index f916ce9e3d8b9f69831141d11f4c96863270b36c..e82990ce7f1f09249c731af952cdd748d0c2ed8b 100644
--- a/Source/core/frame/Window.idl
+++ b/Source/core/frame/Window.idl
@@ -166,7 +166,7 @@
     [Replaceable] readonly attribute Console console;
 
     // cross-document messaging
-    [DoNotCheckSecurity, Custom, RaisesException] void postMessage(SerializedScriptValue message, DOMString targetOrigin, optional MessagePort[] messagePorts);
+    [DoNotCheckSecurity, Custom, RaisesException] void postMessage(SerializedScriptValue message, DOMString targetOrigin, optional sequence<Transferable> transfer);
 
     [Replaceable] readonly attribute Performance performance;
 
@@ -211,6 +211,21 @@
     [Custom, NotEnumerable] getter Window (DOMString name);
 };
 
+// http://www.whatwg.org/specs/web-apps/current-work/#transferable-objects
+//
+// Expressing the Transferable typedef in IDL depends on http://crbug.com/240176.
+// The postMessage() methods taking a Transferable array argument have custom
+// binding code that is able to handle the Transferables that we currently
+// recognize. To be able to declare a postMessage() signature that matches
+// the implementation, we provide a Transferable typedef but with an
+// incomplete type.
+//
+// FIXME: make this typedef accurate once enough of http://crbug.com/240176
+// is in place.
+// FIXME: consider putting this typedef in an .idl file containing spec-wide
+// utility type definitions.
+typedef MessagePort Transferable;
+
 Window implements GlobalEventHandlers;
 Window implements WindowBase64;
 Window implements WindowEventHandlers;