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

Source/core/testing/ReadableStreamTestSource.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 ReadableStreamTestSource_h
+#define ReadableStreamTestSource_h
+
+#include "bindings/core/v8/ScriptState.h"
+#include "bindings/core/v8/ScriptValue.h"
+#include "bindings/core/v8/ScriptWrappable.h"
+#include "core/streams/ReadableStreamController.h"
+#include "core/streams/UnderlyingSourceBase.h"
+#include "platform/heap/Handle.h"
+
+namespace blink {
+
+class ReadableStreamTestSource final : public UnderlyingSourceBase {
+    DEFINE_WRAPPERTYPEINFO();
+
+public:
+    ReadableStreamTestSource(ScriptState* scriptState) : UnderlyingSourceBase(scriptState) { }
+
+    void enqueue(ScriptValue v) { controller()->enqueue(v); }
+    void close() { controller()->close(); }
+    void error(ScriptValue e) { controller()->error(e); }
+};
+
+} // namespace blink
+
+#endif // ReadableStreamTestSource_h