Do not use NavigationEntry to block history navigations.

This is no longer necessary after r477371.

BUG=777419
TEST=See bug for repro steps.

Cq-Include-Trybots: master.tryserver.chromium.linux:linux_site_isolation
Change-Id: I701e4d4853858281b43e3743b12274dbeadfbf18
Reviewed-on: https://chromium-review.googlesource.com/733959
Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Reviewed-by: Nasko Oskov <nasko@chromium.org>
Commit-Queue: Charlie Reis <creis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#511942}
  BrowserSideNavigationBrowserTest() {}

  BrowserSideNavigationBrowserTest() {}
