Bug 1445981. HTMLDialogElement should have HTMLConstructor. r=qdot

MozReview-Commit-ID: IthGCIsB6hr

dom/webidl/HTMLDialogElement.webidl

@@ -11,7 +11,7 @@
  * and create derivative works of this document.
  */
 
-[Pref="dom.dialog_element.enabled"]
+[Pref="dom.dialog_element.enabled", HTMLConstructor]
 interface HTMLDialogElement : HTMLElement {
   [CEReactions, SetterThrows]
   attribute boolean open;

testing/web-platform/meta/custom-elements/builtin-coverage.html.ini

@@ -1,4 +1,6 @@
 [builtin-coverage.html]
+  prefs: [dom.dialog_element.enabled:true]
+
   [bdi: Define a customized built-in element]
     expected: FAIL