Go back and forward through BackForwardController.

So we don't access the deprecated accessors from Page directly.

R=abarth@chromium.org

Review URL: https://codereview.chromium.org/19772014

git-svn-id: svn://svn.chromium.org/blink/trunk@154591 bbb929c8-8fbe-4397-9dbb-9b2b20218538