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

Source/core/testing/ReadableStreamTestCase.idl

Index: Source/core/testing/ReadableStreamTestCase.idl
diff --git a/Source/core/events/CustomEventInit.idl b/Source/core/testing/ReadableStreamTestCase.idl
similarity index 50%
copy from Source/core/events/CustomEventInit.idl
copy to Source/core/testing/ReadableStreamTestCase.idl
index 0c02acd922fcd22ed48dfde37f298ed17d9c958f..495ddc7d89c569909e5f6eb028e61a76c4fc5a74 100644
--- a/Source/core/events/CustomEventInit.idl
+++ b/Source/core/testing/ReadableStreamTestCase.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;
+[GarbageCollected]
+interface ReadableStreamTestCase {
+    readonly attribute ReadableStreamTestRequest request;
+    readonly attribute ReadableStreamTestSource source;
 };