Streams Implementation Update: async read

Source/core/streams/ReadableStream.idl

 // Copyright 2014 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.
 
 [
     RuntimeEnabled=ExperimentalStream,
     GarbageCollected,
     Exposed=(Window,Worker),
     ActiveDOMObject,
 ] interface ReadableStream {
+    [RaisesException] ReadableStreamReader getReader();
     [CallWith=ScriptState] Promise cancel(any reason);
-    [RaisesException] ReadableStreamReader getReader();
-
-    [CallWith=ScriptState] readonly attribute Promise closed;
 };