Implement ReadableStream as a V8 extra

Source/core/streams/ScriptUnderlyingSource.h

+#ifndef ScriptUnderlyingSource_h
+#define ScriptUnderlyingSource_h
+
+#include "bindings/core/v8/ScriptPromise.h"
+#include "bindings/core/v8/ScriptValue.h"
+#include "bindings/core/v8/ScriptWrappable.h"
+#include "core/streams/ScriptReadableStreamController.h"
+
+namespace blink {
+
+// TODO do we need ActiveDOMObject??
+
+class CORE_EXPORT ScriptUnderlyingSource : public GarbageCollectedFinalized<ScriptUnderlyingSource>, public ScriptWrappable {
+    DEFINE_WRAPPERTYPEINFO();
+
+public:
+    ScriptPromise start(ScriptValue stream);
+    ScriptPromise pull();
+    ScriptPromise cancel(ScriptValue reason);
+
+    DECLARE_VIRTUAL_TRACE();
+
+protected:
+    virtual ScriptPromise startImpl() = 0;
+    virtual ScriptPromise pullImpl() = 0;
+    virtual ScriptPromise cancelImpl(ScriptValue reason) = 0;
+
+    // TODO is a reference a good idea?
+    ScriptReadableStreamController& controller() const { return *m_controller; }
+
+private:
+    // TODO should be using a smart pointer of some sort ... Member<>? Persistent<>? RefPtr<>?
+    // ScriptReadableStreamController is not GarbageCollectedFinalized<>... if we made it so, how do we handle it
+    // containing a ScriptValue?
+    ScriptReadableStreamController* m_controller;
+};
+
+} // namespace blink
+
+#endif ScriptUnderlyingSource_h