Bug 1509794 - Add UseCounter telemetry for Custom Elements API; r=smaug

Differential Revision: https://phabricator.services.mozilla.com/D12886

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

dom/base/UseCounters.conf

@@ -132,3 +132,6 @@ custom DocumentOpen calls document.open in a way that creates a new Window objec
 custom DocumentOpenReplace calls document.open in a way that creates a new Window object and replaces the old history entry.
 
 custom FilteredCrossOriginIFrame cross-origin <iframe> within a CSS/SVG filter
+
+// Custom Elements
+method CustomElementRegistry.define

dom/webidl/CustomElementRegistry.webidl

@@ -4,7 +4,7 @@
 
 // https://html.spec.whatwg.org/#dom-window-customelements
 interface CustomElementRegistry {
-  [CEReactions, Throws]
+  [CEReactions, Throws, UseCounter]
   void define(DOMString name, Function functionConstructor,
               optional ElementDefinitionOptions options);
   [ChromeOnly, Throws]