Bug 1381399 - Don't try to close reftest window if it's already close…

…d, r=ato

If the run ended with the reftest window closed e.g. because the final
test timed out, we shouldn't try to close it again, since that results
in an error.

MozReview-Commit-ID: 7FW96EkfgBG

testing/marionette/reftest.js

@@ -128,7 +128,9 @@ min-width: 600px; min-height: 600px; max-width: 600px; max-height: 600px`;
   }
 
   abort() {
-    this.driver.close();
+    if (this.reftestWin) {
+      this.driver.close();
+    }
     this.reftestWin = null;
   }