Fix PromiseRejectionEvent's promise attribute

As discovered in whatwg/streams#1298 (comment), Promise<T> is actually not an appropriate type for it.

source

@@ -105918,19 +105918,25 @@ dictionary <dfn dictionary>ErrorEventInit</dfn> : <span>EventInit</span> {
 interface <dfn interface>PromiseRejectionEvent</dfn> : <span>Event</span> {
   <span data-x="dom-Event-constructor">constructor</span>(DOMString type, <span>PromiseRejectionEventInit</span> eventInitDict);
 
-  readonly attribute Promise&lt;any&gt; <span data-x="dom-PromiseRejectionEvent-promise">promise</span>;
+  readonly attribute <span data-x="idl-object">object</span> <span data-x="dom-PromiseRejectionEvent-promise">promise</span>;
   readonly attribute any <span data-x="dom-PromiseRejectionEvent-reason">reason</span>;
 };
 
 dictionary <dfn dictionary>PromiseRejectionEventInit</dfn> : <span>EventInit</span> {
-  required Promise&lt;any&gt; promise;
+  required <span data-x="idl-object">object</span> promise;
   any reason;
 };</code></pre>
 
   <p>The <dfn attribute for="PromiseRejectionEvent"><code
   data-x="dom-PromiseRejectionEvent-promise">promise</code></dfn> attribute must return the value it
   was initialized to. It represents the promise which this notification is about.</p>
 
+  <p class="note">Because of how Web IDL conversion rules for <code
+  data-x="">Promise&lt;<var>T</var>></code> types always wrap the input into a new promise, the
+  <code data-x="dom-PromiseRejectionEvent-promise">promise</code> attribute is of type <code
+  data-x="idl-object">object</code> instead, which is more appropriate for representing an opaque
+  handle to the original promise object.</p>
+
   <p>The <dfn attribute for="PromiseRejectionEvent"><code
   data-x="dom-PromiseRejectionEvent-reason">reason</code></dfn> attribute must return the value it
   was initialized to. It represents the rejection reason for the promise.</p>