Update interfaces/fetch.idl (#23917)

Source: https://github.com/tidoust/reffy-reports/blob/e5e4f6a/ed/idl/fetch.idl
Build: https://travis-ci.org/tidoust/reffy-reports/builds/693970304
diff --git a/interfaces/fetch.idl b/interfaces/fetch.idl
index 7ff0745..515d02b 100644
--- a/interfaces/fetch.idl
+++ b/interfaces/fetch.idl
@@ -17,7 +17,9 @@
   iterable<ByteString, ByteString>;
 };
 
-typedef (Blob or BufferSource or FormData or URLSearchParams or ReadableStream or USVString) BodyInit;
+typedef (Blob or BufferSource or FormData or URLSearchParams or USVString) XMLHttpRequestBodyInit;
+
+typedef (ReadableStream or XMLHttpRequestBodyInit) BodyInit;
 interface mixin Body {
   readonly attribute ReadableStream? body;
   readonly attribute boolean bodyUsed;