Merge FrameLoader::loadWithNavigationAction and checkNavigationPolicyAndContinueLoad

Remove some cruft from the merged function

BUG=none

Review URL: https://chromiumcodereview.appspot.com/21002004

git-svn-id: svn://svn.chromium.org/blink/trunk@155804 bbb929c8-8fbe-4397-9dbb-9b2b20218538