Clean up and deflake history navigation test

When returning from having navigated to navigate-back.html, both
runSecondStageOfTest() and the popstate event handler would set a
timeout that called history.back().  The first navigation would
trigger the hashchange event handler that signaled a pass, the
second ought to have had no effect.

In practice, the test occasionally timed out or crashed instead,
for unknown reasons.  Removing the extra history.back() timeout
apparently avoids this.

BUG=339265

Review URL: https://codereview.chromium.org/148453014

git-svn-id: svn://svn.chromium.org/blink/trunk@166325 bbb929c8-8fbe-4397-9dbb-9b2b20218538