DocumentLoader NavigationPolicy and SubstituteData cleanup

Remove the unused SubstituteData::responseURL() and its references in DocumentLoader

Remove unused case where we set a triggering action if one isn't present.

Remove an unused case where we override the load type of SubstituteData for a failing url in the back/forward case.

BUG=none

Review URL: https://chromiumcodereview.appspot.com/22618002

git-svn-id: svn://svn.chromium.org/blink/trunk@155783 bbb929c8-8fbe-4397-9dbb-9b2b20218538