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

Source/core/testing/Internals.idl

Index: Source/core/testing/Internals.idl
diff --git a/Source/core/testing/Internals.idl b/Source/core/testing/Internals.idl
index 4ee95f627445738bad9204c4be35fe49174c723d..1fbf2132f8d47888deac3ebe1d297157e31a047a 100644
--- a/Source/core/testing/Internals.idl
+++ b/Source/core/testing/Internals.idl
@@ -28,6 +28,7 @@
     DoNotCheckConstants,
     GarbageCollected,
     TypeChecking=Interface,
+    ConstructorCallWith=ScriptState,
 ] interface Internals {
     DOMString address(Node node);
 
@@ -319,4 +320,6 @@
     [Unscopeable] DOMString unscopeableMethod();
 
     ClientRectList focusRingRects(Element element);
+
+    readonly attribute any readableStream;
 };