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

Source/core/streams/UnderlyingSourceBase.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 UnderlyingSourceBase_h
+#define UnderlyingSourceBase_h
+
+#include "bindings/core/v8/ScriptPromise.h"
+#include "bindings/core/v8/ScriptValue.h"
+#include "bindings/core/v8/ScriptWrappable.h"
+#include "core/CoreExport.h"
+#include "core/dom/ActiveDOMObject.h"
+#include "platform/heap/Handle.h"
+
+namespace blink {
+
+class ReadableStreamController;
+
+class CORE_EXPORT UnderlyingSourceBase : public GarbageCollectedFinalized<UnderlyingSourceBase>, public ScriptWrappable, public ActiveDOMObject {
+    DEFINE_WRAPPERTYPEINFO();
+
+public:
+    DECLARE_VIRTUAL_TRACE();
+    virtual ~UnderlyingSourceBase() { }
+
+    ScriptPromise startWrapper(ScriptState*, ScriptValue stream);
+    virtual ScriptPromise start(ScriptState*);
+
+    virtual ScriptPromise pull(ScriptState*);
+
+    ScriptPromise cancelWrapper(ScriptState*, ScriptValue reason);
+    virtual ScriptPromise cancel(ScriptState*, ScriptValue reason);
+
+    // ActiveDOMObject
+    virtual bool hasPendingActivity() const override;
+
+protected:
+    UnderlyingSourceBase(ScriptState* scriptState) : ActiveDOMObject(scriptState->executionContext())
+    {
+        this->suspendIfNeeded();
+    }
+
+    ReadableStreamController* controller() { return m_controller; }
+
+private:
+    Member<ReadableStreamController> m_controller;
+};
+
+} // namespace blink
+
+#endif // UnderlyingSourceBase_h