Factor out preflight result handling code in DocumentThreadableLoader::didReceiveResponse()

It's big enough to be placed in a separate method.

BUG=none

Review URL: https://codereview.chromium.org/197203002

git-svn-id: svn://svn.chromium.org/blink/trunk@174583 bbb929c8-8fbe-4397-9dbb-9b2b20218538