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

Source/core/streams/UnderlyingSourceBase.cpp

+// 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.
+
+#include "config.h"
+#include "core/streams/UnderlyingSourceBase.h"
+
+#include <v8.h>
+
+namespace blink {
+
+ScriptPromise UnderlyingSourceBase::startWrapper(ScriptState* scriptState, ScriptValue stream)
+{
+    // In ReadableStream.js, we special-case externally-controlled streams by having them pass themselves to start
+    // as the first argument. This allows us to create a ScriptReadableStreamController.
+
+    // TODO pointer usage here is presumably very borked
+    m_controller = new ReadableStreamController(stream);
+
+    return start(scriptState);
+}
+
+ScriptPromise UnderlyingSourceBase::start(ScriptState* scriptState)
+{
+    return ScriptPromise::cast(scriptState, v8::Undefined(scriptState->isolate()));
+}
+
+ScriptPromise UnderlyingSourceBase::pull(ScriptState* scriptState)
+{
+    return ScriptPromise::cast(scriptState, v8::Undefined(scriptState->isolate()));
+}
+
+ScriptPromise UnderlyingSourceBase::cancel(ScriptState* scriptState, ScriptValue reason)
+{
+    return ScriptPromise::cast(scriptState, v8::Undefined(scriptState->isolate()));
+}
+
+DEFINE_TRACE(UnderlyingSourceBase)
+{
+}
+
+} // namespace blink