[imports] credentials flag bits need to be updated to current fetch terminology

[Boris Zbarsky]

In http://w3c.github.io/webcomponents/spec/imports/#fetching-import the 
spec says:

  Fetch a resource from LOCATION with request's origin set to the
  origin of the master document, the mode to CORS and the omit
  credentials mode to CORS.
There is no "omit credentials" mode in the current Fetch draft, and the 
mode that _is_ there, "credentials mode", doesn't have "CORS" as a 
value.  Presumably this is asking for "same-origin"?

-Boris