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

Source/core/testing/ReadableStreamTestRequest.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 ReadableStreamTestRequest_h
+#define ReadableStreamTestRequest_h
+
+#include "bindings/core/v8/ScriptState.h"
+#include "bindings/core/v8/ScriptValue.h"
+#include "bindings/core/v8/ScriptWrappable.h"
+#include "platform/heap/Handle.h"
+
+namespace blink {
+
+class UnderlyingSourceBase;
+
+class ReadableStreamTestRequest final : public GarbageCollected<ReadableStreamTestRequest>, public ScriptWrappable {
+    DEFINE_WRAPPERTYPEINFO();
+
+public:
+    DEFINE_INLINE_TRACE() { }
+
+    ReadableStreamTestRequest(ScriptState*, UnderlyingSourceBase*);
+
+    const char* metadata() { return "some metadata"; }
+    ScriptValue body(ScriptState*);
+};
+
+} // namespace blink
+
+#endif // ReadableStreamTestRequest_h