Remove a bogus assert in restoreScrollPositionAndViewState.

iframes initially populated by javascript urls can cause us to call
restoreDocumentState() without a current history item.

BUG=303279

Review URL: https://codereview.chromium.org/25790002

git-svn-id: svn://svn.chromium.org/blink/trunk@158952 bbb929c8-8fbe-4397-9dbb-9b2b20218538