Bug 1463739 - [marionette] Move check for loaded state of reftest window to base reftest code. r=jgraham

While using openDialog() with the extra parameters for passing
in the callback to resolve is fine, it adds extra complexity.
The base reftest code in Marionette itself should better handle
that.

MozReview-Commit-ID: AtOV3LevK1c

--HG--
extra : rebase_source : 2a32617abf2e33d520b37d186285db6d04657965

testing/marionette/reftest.js

@@ -95,13 +95,13 @@ reftest.Runner = class {
   }
 
   async openWindow() {
-    let reftestWin;
+    let reftestWin = this.parentWindow.open(
+        "chrome://marionette/content/reftest.xul",
+        "reftest",
+        "chrome,dialog,height=600,width=600");
+
     await new Promise(resolve => {
-      reftestWin = this.parentWindow.openDialog(
-          "chrome://marionette/content/reftest.xul",
-          "reftest",
-          "chrome,dialog,height=600,width=600,all",
-          resolve);
+      reftestWin.addEventListener("load", resolve, {once: true});
     });
 
     let browser = reftestWin.document.createElementNS(XUL_NS, "xul:browser");

testing/marionette/reftest.xul

@@ -1 +1,2 @@
-<window id="reftest" xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul" onload="window.arguments[0]()"></window>
+<window id="reftest" xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul">
+</window>