Remove the last remnants of client redirect callbacks

A couple of variables on DocumentLoader, exposed via WebDataSource, serve the
same purpose.

BUG=261374

Review URL: https://chromiumcodereview.appspot.com/21639002

git-svn-id: svn://svn.chromium.org/blink/trunk@155405 bbb929c8-8fbe-4397-9dbb-9b2b20218538