ReadableStream.cancel's parameter should be optional.

Source/core/streams/ReadableStreamReader.h

 // 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.
 
 #ifndef ReadableStreamReader_h
 #define ReadableStreamReader_h
 
 #include "bindings/core/v8/ScriptPromise.h"
 #include "bindings/core/v8/ScriptPromiseProperty.h"
 #include "bindings/core/v8/ScriptValue.h"
@@ skipping 13 matching lines @@
 // https://streams.spec.whatwg.org/.
 class ReadableStreamReader final : public GarbageCollectedFinalized<ReadableStreamReader>, public ScriptWrappable, public ActiveDOMObject {
     DEFINE_WRAPPERTYPEINFO();
     WILL_BE_USING_GARBAGE_COLLECTED_MIXIN(ReadableStreamReader);
 public:
     // The stream must not be locked to any ReadableStreamReader when called.
     ReadableStreamReader(ReadableStream*);
 
     ScriptPromise closed(ScriptState*);
     bool isActive() const;
+    ScriptPromise cancel(ScriptState*);
     ScriptPromise cancel(ScriptState*, ScriptValue reason);
     ScriptPromise read(ScriptState*);
     void releaseLock(ExceptionState&);
     void releaseLock();
 
     bool hasPendingActivity() const override;
     void stop() override;
 
     DECLARE_TRACE();
 
 private:
     using ClosedPromise = ScriptPromiseProperty<Member<ReadableStreamReader>, ToV8UndefinedGenerator, RefPtrWillBeMember<DOMException>>;
 
     const Member<ReadableStream> m_stream;
     ReadableStream::State m_stateAfterRelease;
     Member<ClosedPromise> m_closed;
 };
 
 } // namespace blink
 
 #endif // ReadableStreamReader_h