Simplify setting lockBackForwardList() in NavigationScheduler.

Remove ScheduledNavigation::didStartTimer() and
HistoryController::currentItemShouldBeReplaced(). Replace
then with a new state on FrameLoaderStateMachine and inline
the new check in
NavigationScheduler::mustLockBackForwardList().

BUG=321280

Review URL: https://codereview.chromium.org/76653002

git-svn-id: svn://svn.chromium.org/blink/trunk@162384 bbb929c8-8fbe-4397-9dbb-9b2b20218538