Return _getReviewedFiles promise to prevent its loss

Change-Id: Ib9b3fa99ba875c905806d378977f66e74c6364d9
diff --git a/polygerrit-ui/app/elements/change/gr-file-list/gr-file-list.js b/polygerrit-ui/app/elements/change/gr-file-list/gr-file-list.js
index 947d982..39e50a9 100644
--- a/polygerrit-ui/app/elements/change/gr-file-list/gr-file-list.js
+++ b/polygerrit-ui/app/elements/change/gr-file-list/gr-file-list.js
@@ -64,7 +64,7 @@
       }).then(function(loggedIn) {
         if (!loggedIn) { return; }
 
-        _this._getReviewedFiles().then(function(reviewed) {
+        return _this._getReviewedFiles().then(function(reviewed) {
           _this._reviewed = reviewed;
         });
       }));