Observe change object to handle refresh

The "ready()" function isn't quite right; it has been noted that when
you navigate between changes the Zuul Summary tab isn't updating and
you need a full refresh.

I think what we should be doing is observing the change property and
building the table in that handler.

Change-Id: Id3410c28d2cd8fdb08fc14d755c43f1d2015eaa5

zuul-results-summary/zuul-results-summary.js

diff --git a/zuul-results-summary/zuul-results-summary.js b/zuul-results-summary/zuul-results-summary.js
index 9e3f687..ec92202 100644
--- a/zuul-results-summary/zuul-results-summary.js
+++ b/zuul-results-summary/zuul-results-summary.js
@@ -26,7 +26,10 @@
     */
   static get properties() {
     return {
-      change: Object,
+      change: {
+        type: Object,
+        observer: '_changeChanged'
+      },
       revision: Object,
     };
   }
@@ -148,10 +151,8 @@
     return [status, pipeline];
   }
 
-  /** ready function */
-  ready() {
-    super.ready();
-
+  /** Change Modified */
+  _changeChanged(change, oldChange) {
     /*
      * change-view-tab-content gets passed ChangeInfo object [1],
      * registered in the property "change".  We walk the list of
@@ -180,7 +181,7 @@
      * [1] https://gerrit-review.googlesource.com/Documentation/rest-api-changes.html#change-info
      */
     this.__table = [];
-    this.change.messages.forEach(message => {
+    change.messages.forEach(message => {
       if (! (this._match_message_via_tag(message) ||
                     this._match_message_via_regex(message))) {
         return;