ReadableStream.cancel's parameter should be optional.

Source/core/streams/ReadableStreamReader.cpp

 // 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.
 
 #include "config.h"
 #include "core/streams/ReadableStreamReader.h"
 
 #include "bindings/core/v8/ExceptionState.h"
 #include "bindings/core/v8/ScriptPromiseResolver.h"
 #include "bindings/core/v8/V8IteratorResultValue.h"
@@ skipping 22 matching lines @@
 ScriptPromise ReadableStreamReader::closed(ScriptState* scriptState)
 {
     return m_closed->promise(scriptState->world());
 }
 
 bool ReadableStreamReader::isActive() const
 {
     return m_stream->isLockedTo(this);
 }
 
+ScriptPromise ReadableStreamReader::cancel(ScriptState* scriptState)
+{
+    return cancel(scriptState, ScriptValue(scriptState, v8::Undefined(scriptState->isolate())));
+}
+
 ScriptPromise ReadableStreamReader::cancel(ScriptState* scriptState, ScriptValue reason)
 {
     if (isActive())
         return m_stream->cancelInternal(scriptState, reason);
 
     // A method should return a different promise on each call.
     RefPtrWillBeRawPtr<ScriptPromiseResolver> resolver = ScriptPromiseResolver::create(scriptState);
     ScriptPromise promise = resolver->promise();
     resolver->resolve(closed(scriptState).v8Value());
     return promise;
@@ skipping 66 matching lines @@
 }
 
 DEFINE_TRACE(ReadableStreamReader)
 {
     visitor->trace(m_stream);
     visitor->trace(m_closed);
     ActiveDOMObject::trace(visitor);
 }
 
 } // namespace blink