Add methods for creating V8 extras-based ReadableStreams from C++

Source/core/testing/ReadableStreamTestCase.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/testing/ReadableStreamTestCase.h"
+
+#include "core/testing/ReadableStreamTestRequest.h"
+#include "core/testing/ReadableStreamTestSource.h"
+
+namespace blink {
+
+ReadableStreamTestCase::ReadableStreamTestCase(ScriptState* scriptState)
+    : m_source(new ReadableStreamTestSource(scriptState))
+    , m_request(new ReadableStreamTestRequest(scriptState, m_source))
+{
+}
+
+DEFINE_TRACE(ReadableStreamTestCase)
+{
+    visitor->trace(m_request);
+    visitor->trace(m_source);
+}
+
+}