Greatly trim down what HistoryController::restoreDocumentState does.

It has a lot of complex rules that we don't use. We just want to restore form
state if there's form state saved and it's a back/forward navigation.

BUG=none

Review URL: https://chromiumcodereview.appspot.com/24787004

git-svn-id: svn://svn.chromium.org/blink/trunk@158404 bbb929c8-8fbe-4397-9dbb-9b2b20218538