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

Source/core/streams/UnderlyingSourceBase.h

+#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/streams/ReadableStreamController.h"
+
+namespace blink {
+
+// TODO do we need ActiveDOMObject??
+
+class CORE_EXPORT UnderlyingSourceBase : public GarbageCollectedFinalized<UnderlyingSourceBase>, public ScriptWrappable {
+    DEFINE_WRAPPERTYPEINFO();
+
+public:
+    ScriptPromise startWrapper(ScriptState*, ScriptValue stream);
+    virtual ScriptPromise start(ScriptState*);
+    virtual ScriptPromise pull(ScriptState*);
+    virtual ScriptPromise cancel(ScriptState*, ScriptValue reason);
+
+    DECLARE_VIRTUAL_TRACE();
+
+protected:
+    // TODO is a reference a good idea?
+    ReadableStreamController& controller() const { return *m_controller; }
+
+private:
+    // TODO should be using a smart pointer of some sort ... Member<>? Persistent<>? RefPtr<>?
+    // ReadableStreamController is not GarbageCollectedFinalized<>... if we made it so, how do we handle it
+    // containing a ScriptValue?
+    ReadableStreamController* m_controller;
+};
+
+} // namespace blink
+
+#endif UnderlyingSourceBase_h