Remove BackForwardClient, cleanup starting a history navigation

* Don't proxy history navigations through the main frame, there's no need
* Have Source/core interact with history state entirely through FrameLoaderClient. It doesn't need to know that FrameLoaderClientImpl is actually talking to WebViewClient (though that will need to change eventually)
* Remove NavigationScheduler::scheduleHistoryNavigation. History navigations are always asynchronous anyway, since they need to talk to the browser process.

BUG=

Review URL: https://codereview.chromium.org/669013002

git-svn-id: svn://svn.chromium.org/blink/trunk@184989 bbb929c8-8fbe-4397-9dbb-9b2b20218538