ReadableStream.cancel's parameter should be optional.

Source/core/streams/ReadableStreamReader.idl

 // 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.
 
 [
     RuntimeEnabled=ExperimentalStream,
     GarbageCollected,
     ActiveDOMObject,
     Exposed=(Window,Worker),
 ] interface ReadableStreamReader {
     // FIXME: Add constructor.
 
     [CallWith=ScriptState] readonly attribute Promise<void> closed;
     [CallWith=ScriptState] Promise<any> read();
 
-    [CallWith=ScriptState] Promise<void> cancel(any reason);
+    [CallWith=ScriptState] Promise<void> cancel(optional any reason);
     [RaisesException] void releaseLock();
 };