Bug 1574195 part 3. Switch XPathEvaluator to using IDL mixins. r=peterv

Differential Revision: https://phabricator.services.mozilla.com/D46522

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

dom/webidl/Document.webidl

@@ -618,7 +618,7 @@ partial interface Document {
   [Func="Document::DocumentSupportsL10n"] readonly attribute DocumentL10n? l10n;
 };
 
-Document implements XPathEvaluator;
+Document includes XPathEvaluatorMixin;
 Document includes GlobalEventHandlers;
 Document includes DocumentAndElementEventHandlers;
 Document implements TouchEventHandlers;

dom/webidl/XPathEvaluator.webidl

@@ -6,7 +6,10 @@
 
 interface XPathEvaluator {
   constructor();
+};
+XPathEvaluator includes XPathEvaluatorMixin;
 
+interface mixin XPathEvaluatorMixin {
   [NewObject, Throws]
   XPathExpression createExpression(DOMString expression,
                                    optional XPathNSResolver? resolver = null);