[css-shared-element-transitions-1] Align with Web IDL specification (w3c#7706)

Author: autokagami

css-shared-element-transitions-1/Overview.bs

@@ -441,6 +441,7 @@ callback UpdateDOMCallback = Promise<any> ();
 ## The {{DOMTransition}} interface ## {#the-domtransition-interface}
 
 <script type=idl>
+[Exposed=Window]
 interface DOMTransition {
     undefined skipTransition();
     readonly attribute Promise<undefined> finished;