Use (ArrayBufferView or undefined) for ReadableStreamBYOBReadResult

The standard returns an ReadableStreamBYOBReadResult with an undefined
value, and the tests rely on this, but it wasn't permitted by the IDL. Fix the IDL
to match the actual behaviour.

Closes whatwg/webidl#1094.

Author: saschanaz

index.bs

@@ -1308,7 +1308,7 @@ interface ReadableStreamBYOBReader {
 ReadableStreamBYOBReader includes ReadableStreamGenericReader;
 
 dictionary ReadableStreamBYOBReadResult {
-  ArrayBufferView value;
+  (ArrayBufferView or undefined) value;
   boolean done;
 };
 </xmp>

reference-implementation/lib/ReadableStreamBYOBReadResult.webidl

@@ -1,4 +1,4 @@
 dictionary ReadableStreamBYOBReadResult {
-  ArrayBufferView value;
+  (ArrayBufferView or undefined) value;
   boolean done;
 };