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

Source/core/testing/ReadableStreamTestSource.idl

Index: Source/core/testing/ReadableStreamTestSource.idl
diff --git a/Source/core/events/CustomEventInit.idl b/Source/core/testing/ReadableStreamTestSource.idl
similarity index 56%
copy from Source/core/events/CustomEventInit.idl
copy to Source/core/testing/ReadableStreamTestSource.idl
index 0c02acd922fcd22ed48dfde37f298ed17d9c958f..879951ab23ca38671fd152c15e7a51b74d3c4842 100644
--- a/Source/core/events/CustomEventInit.idl
+++ b/Source/core/testing/ReadableStreamTestSource.idl
@@ -2,8 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// https://dom.spec.whatwg.org/#interface-customevent
-
-dictionary CustomEventInit : EventInit {
-    any detail = null;
+interface ReadableStreamTestSource : UnderlyingSourceBase {
+    void enqueue(any chunk);
+    void close();
+    void error(any e);
 };