Show reviewers in change log

Recognize reviewer_updates from change/detail REST API response and
display inline with the messages and comments.

Change-Id: Iaa8740aeb5a194cc548cce777c9cef79cad652a2
diff --git a/polygerrit-ui/app/elements/change/gr-change-view/gr-change-view.js b/polygerrit-ui/app/elements/change/gr-change-view/gr-change-view.js
index 700f162..76874c0 100644
--- a/polygerrit-ui/app/elements/change/gr-change-view/gr-change-view.js
+++ b/polygerrit-ui/app/elements/change/gr-change-view/gr-change-view.js
@@ -552,6 +552,9 @@
               function(change) {
                 // Issue 4190: Coalesce missing topics to null.
                 if (!change.topic) { change.topic = null; }
+                if (!change.reviewer_updates) {
+                  change.reviewer_updates = null;
+                }
                 this._change = change;
               }.bind(this));
     },