Replace usage of 'addedKeys' under splices in _reviewersChanged

This property was removed under Polymer 2.

Bug: Issue 11299
Change-Id: I81ae6a95ffb891ab28149a2bfd4c6ee90bac2fdf
diff --git a/polygerrit-ui/app/elements/change/gr-reply-dialog/gr-reply-dialog.js b/polygerrit-ui/app/elements/change/gr-reply-dialog/gr-reply-dialog.js
index 640cbc8..f6687b6 100644
--- a/polygerrit-ui/app/elements/change/gr-reply-dialog/gr-reply-dialog.js
+++ b/polygerrit-ui/app/elements/change/gr-reply-dialog/gr-reply-dialog.js
@@ -331,8 +331,8 @@
         let account;
         // Remove any accounts that already exist as a CC.
         for (const splice of splices.indexSplices) {
-          for (const addedKey of splice.addedKeys) {
-            account = this.get(`_reviewers.${addedKey}`);
+          for (let i = 0; i < splice.addedCount; i++) {
+            account = splice.object[splice.index + i];
             key = this._accountOrGroupKey(account);
             index = this._ccs.findIndex(
                 account => this._accountOrGroupKey(account) === key);