Cleanup in NavigationScheduler

* cancel() and clear() are identical. Standardize on cancel().
* schededuleRefresh() is confusing, as it has nothing to do
with the http refresh header, which goes through
scheduleRedirect(). Rename it to scheduleReload().
* Per http://www.whatwg.org/specs/web-apps/current-work/multipage/browsers.html#dom-history-go,
history.go(0) is identical to location.reload(). Remove the
special case in ScheduledHistoryNavigation::fire() and use
ScheduledReload for the 0 case instead.

BUG=

Review URL: https://codereview.chromium.org/436973002

git-svn-id: svn://svn.chromium.org/blink/trunk@179430 bbb929c8-8fbe-4397-9dbb-9b2b20218538