ReadableStream.cancel's parameter should be optional.

Source/core/streams/ReadableStream.cpp

 // 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.
 
 #include "config.h"
 #include "core/streams/ReadableStream.h"
 
 #include "bindings/core/v8/ExceptionState.h"
 #include "bindings/core/v8/ScriptFunction.h"
 #include "bindings/core/v8/ScriptPromiseResolver.h"
@@ skipping 75 matching lines @@
 }
 
 void ReadableStream::readInternalPostAction()
 {
     ASSERT(m_state == Readable);
     if (isQueueEmpty() && m_isDraining)
         closeInternal();
     callPullIfNeeded();
 }
 
+ScriptPromise ReadableStream::cancel(ScriptState* scriptState)
+{
+    return cancel(scriptState, ScriptValue(scriptState, v8::Undefined(scriptState->isolate())));
+}
+
 ScriptPromise ReadableStream::cancel(ScriptState* scriptState, ScriptValue reason)
 {
     if (m_reader)
         return ScriptPromise::reject(scriptState, V8ThrowException::createTypeError(scriptState->isolate(), "this stream is locked to a ReadableStreamReader"));
     if (m_state == Closed)
         return ScriptPromise::cast(scriptState, v8::Undefined(scriptState->isolate()));
     if (m_state == Errored)
         return ScriptPromise::rejectWithDOMException(scriptState, m_exception);
 
     return cancelInternal(scriptState, reason);
@@ skipping 75 matching lines @@
 
 DEFINE_TRACE(ReadableStream)
 {
     visitor->trace(m_source);
     visitor->trace(m_exception);
     visitor->trace(m_reader);
     ActiveDOMObject::trace(visitor);
 }
 
 } // namespace blink