Bug 1263887 - Update the active child on a menulist when setting selection. r=enndeakin+6102

MozReview-Commit-ID: KsXLd6ICr0B

--HG--
extra : rebase_source : e501425041754d6182c3d67ea4090394d097358f

toolkit/content/widgets/menulist.xml

@@ -254,6 +254,8 @@
             event.initEvent("ValueChange", true, true);
             this.dispatchEvent(event);
 
+            this.menuBoxObject.activeChild = this.mSelectedInternal;
+
             return val;
           ]]>
         </setter>