Use requestUpdate to trigger an update

Polymer required assigning a new value to the property but in Lit
we can simply request a new update.

Release-Notes: skip
Change-Id: Ia7f7931d05988de8dd6234a302848edd1ba14444
diff --git a/polygerrit-ui/app/elements/change/gr-reply-dialog/gr-reply-dialog.ts b/polygerrit-ui/app/elements/change/gr-reply-dialog/gr-reply-dialog.ts
index a52960a..d9da814 100644
--- a/polygerrit-ui/app/elements/change/gr-reply-dialog/gr-reply-dialog.ts
+++ b/polygerrit-ui/app/elements/change/gr-reply-dialog/gr-reply-dialog.ts
@@ -1597,8 +1597,7 @@
       });
     }
 
-    // Ensure that Polymer picks up the change.
-    this.newAttentionSet = new Set(this.newAttentionSet);
+    this.requestUpdate();
   }
 
   computeHasNewAttention(account?: AccountInfo) {