DigitalCredential's data should be marked at SameObject (#199)

This is mostly just a small optimization, but the .data should have been marked as SameObject, as it never changes.

index.html

@@ -300,7 +300,7 @@ <h2>
     [Exposed=Window, SecureContext]
     interface DigitalCredential : Credential {
       readonly attribute DOMString protocol;
-      readonly attribute object data;
+      [SameObject] readonly attribute object data;
     };
     </pre>
     <p>