Bug 1457336: Mark a few assertions as expected in test_bug451540.xul. rs=mikedeboer

This series exposes an existing bug in nsFindIterator where it cannot rewind
into NAC properly. This nested iterator setup is going to change heavily as part
of making it work on Shadow DOM, so just punt on this for now.

MozReview-Commit-ID: Aq8XfXjQw6f

toolkit/content/tests/chrome/test_bug451540.xul

@@ -30,6 +30,7 @@ https://bugzilla.mozilla.org/show_bug.cgi?id=451540
 
       /** Test for Bug 451540 **/
       SimpleTest.waitForExplicitFinish();
+      SimpleTest.expectAssertions(18, 18); // Will be fixed as part of bug 1455891.
       window.open("bug451540_window.xul", "451540test",
                   "chrome,width=600,height=600");