De-flake http/tests/navigation/back-to-get-after-post.php

I goofed on my setTimeout() syntax because I am bad at javascript.

BUG=340277

Review URL: https://codereview.chromium.org/203743005

git-svn-id: svn://svn.chromium.org/blink/trunk@170886 bbb929c8-8fbe-4397-9dbb-9b2b20218538