Bug 1547463 Explicit load customElements.js where necessary r=bgrins

commonDialog.xul and profileDowngrade.xul both may load early enough
during startup that they don't automatically get customElements.js.
The quick workaround here is just to load it explicitly in those documents.

Differential Revision: https://phabricator.services.mozilla.com/D29235

--HG--
extra : moz-landing-system : lando

toolkit/components/prompts/content/commonDialog.xul

@@ -18,6 +18,7 @@
 
   <script src="chrome://global/content/commonDialog.js"/>
   <script src="chrome://global/content/globalOverlay.js"/>
+  <script src="chrome://global/content/customElements.js"/>
   <script>
     document.addEventListener("DOMContentLoaded", function() {
       commonDialogOnLoad();

toolkit/profile/content/profileDowngrade.xul

@@ -28,6 +28,7 @@
         buttons="accept,extra1" buttonpack="end">
 
   <script src="profileDowngrade.js"/>
+  <script src="chrome://global/content/customElements.js"/>
 
   <hbox flex="1" align="start">
     <image id="info" role="presentation"/>