Fix scroll position restore when doing a same-document back/forward navigation.

We're incorrectly saving the state, and completely failing to restore.

BUG=304249
TEST=http/tests/navigation/same-document-scroll-position-restore.html

Review URL: https://codereview.chromium.org/99133005

git-svn-id: svn://svn.chromium.org/blink/trunk@163564 bbb929c8-8fbe-4397-9dbb-9b2b20218538