messageTray: Defer creation of mainIcon

We need to make sure the Source is fully constructed before creating
mainIcon, as SourceActor will query the count of the source.

This fixes Telepathy Chats.

https://bugzilla.gnome.org/show_bug.cgi?id=682045

js/ui/messageTray.js

@@ -1132,9 +1132,6 @@ const Source = new Lang.Class({
         this.isMuted = false;
 
         this.notifications = [];
-
-        this.mainIcon = new SourceActor(this, this.ICON_SIZE);
-        this._setSummaryIcon(this.createIcon(this.ICON_SIZE));
     },
 
     get count() {
@@ -1178,10 +1175,23 @@ const Source = new Lang.Class({
                              icon_size: size });
     },
 
+    _ensureMainIcon: function(icon) {
+        if (this._mainIcon)
+            return false;
+
+        if (!icon)
+            icon = this.createIcon(this.ICON_SIZE);
+
+        this._mainIcon = new SourceActor(this, this.ICON_SIZE);
+        this._mainIcon.setIcon(icon);
+        return true;
+    },
+
     // Unlike createIcon, this always returns the same actor;
     // there is only one summary icon actor for a Source.
     getSummaryIcon: function() {
-        return this.mainIcon.actor;
+        this._ensureMainIcon();
+        return this._mainIcon.actor;
     },
 
     pushNotification: function(notification) {
@@ -1228,7 +1238,8 @@ const Source = new Lang.Class({
 
     //// Protected methods ////
     _setSummaryIcon: function(icon) {
-        this.mainIcon.setIcon(icon);
+        if (!this._ensureMainIcon(icon))
+            this._mainIcon.setIcon(icon);
     },
 
     open: function(notification) {