Introduce the navigation params struct

Closes whatwg#5727. Along the way this includes some minor fixes and cleanups
which this refactoring made more obvious or necessary:

* Stops unnecessarily passing along the incumbent navigation origin and
  active document navigation origin. They are used early in the
  navigation process and don't need to be threaded through the later
  algorithms (and thus aren't included in the navigation params struct).

* Ensures that all error cases (i.e. "Page load processing model for
  inline content that doesn't have a DOM") result in an opaque-origin
  Document. Previously only one call site set that, and did so
  awkwardly, from a distance.

* Fixes error cases to use "unsafe-none" COOP, instead of null COOP
  (which is not a thing).

* Explicitly passes the browsing context to "Page load processing model
  for inline content that doesn't have a DOM" (which makes it more clear
  that the browsing context is the only input; the navigation params are
  not used in this case).

* Adds two XXX boxes where the current architecture is not quite right:
  multipart/x-mixed-replace needs to use the navigation params somehow,
  and error pages probably need to synthesize a response instead of
  passing null to "create and initialize a Document object".

* Minor algorithm flow cleanups, e.g. using numbered steps instead of
  long sentences with several "and"s.