- route_id related plumbing should go away once we start using per-frame mojo pipe for loading ( issue 712913 )

- other code should be moved into LocalFrame + FrameFetchContext plumbing or moved around CreateURLLoader()

See also: 

Loading arch plan
https://docs.google.com/document/d/10yFcHyq4ddheoJrEmyLMnCkzYvI7ioayPwH4ivDXTwg/edit#

Onion Soup 2.0
https://docs.google.com/document/d/1hBKAYGGM2Zm7qhqXtdLSubklVDKUo9ir-vJtPEOx11Y/edit