Speculative fix for chromevox_tests

TBR=dmazzoni@chromium.org

Bug: 881845
Change-Id: I4b95fe46c6d5c07ad94b17eaf76257e81767763d
Reviewed-on: https://chromium-review.googlesource.com/1225984Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Reviewed-by: David Tseng <dtseng@chromium.org>
Commit-Queue: Dominic Mazzoni <dmazzoni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#591535}

chrome/browser/resources/chromeos/chromevox/testing/chromevox_next_e2e_test_base.js

@@ -64,6 +64,9 @@ ChromeVoxNextE2ETest.prototype = {
         if (evt.target.root.url != url)
           return;
 
+        if (!evt.target.root.docLoaded)
+          return;
+
         r.removeEventListener('focus', listener, true);
         r.removeEventListener('loadComplete', listener, true);
         CommandHandler.onCommand('nextObject');