ReadableStream.cancel's parameter should be optional.

Source/core/streams/ReadableStream.h

 // 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.
 
 #ifndef ReadableStream_h
 #define ReadableStream_h
 
 #include "bindings/core/v8/ScriptPromise.h"
 #include "bindings/core/v8/ScriptPromiseProperty.h"
 #include "bindings/core/v8/ScriptState.h"
@@ skipping 30 matching lines @@
     virtual ~ReadableStream();
 
     bool isStarted() const { return m_isStarted; }
     bool isDraining() const { return m_isDraining; }
     bool isPulling() const { return m_isPulling; }
     State stateInternal() const { return m_state; }
     DOMException* storedException() { return m_exception.get(); }
 
     virtual ScriptPromise read(ScriptState*) = 0;
     ScriptPromise cancel(ScriptState*, ScriptValue reason);
+    ScriptPromise cancel(ScriptState*);
     ScriptPromise cancelInternal(ScriptState*, ScriptValue reason);
 
     virtual bool hasPendingReads() const = 0;
     virtual void resolveAllPendingReadsAsDone() = 0;
     virtual void rejectAllPendingReads(PassRefPtrWillBeRawPtr<DOMException>) = 0;
 
     void close();
     void error(PassRefPtrWillBeRawPtr<DOMException>);
 
     void didSourceStart();
@@ skipping 34 matching lines @@
     bool m_isPulling;
     State m_state;
 
     RefPtrWillBeMember<DOMException> m_exception;
     Member<ReadableStreamReader> m_reader;
 };
 
 } // namespace blink
 
 #endif // ReadableStream_h