Pass a top-level navigation initiator origin to Fetch

[bvandersloot-mozilla]

To un-logjam the cookie layering work, I've started whatwg/fetch#1807. That depends on this info to be piped into Fetch so we can actually specify in WHATWG what SameSite=Strict means.

This patch plumbs that through on top-level navigatable fetches.

This doesn't build because it relies upon the corresponding patch in Fetch. Let me know to land these.

• At least two implementers are interested (and none opposed):
  • Mozilla
  • Apple? (@annevk)
  • Google? (@johannhof )
• Tests are written and can be reviewed and commented upon at:
  • change is not observable, not needed?
• Implementation bugs are filed:
  • change is to update spec interfacing, not needed?
• Corresponding HTML AAM & ARIA in HTML issues & PRs:
• MDN issue is filed:
  • I don't think this is needed?
• The top of this comment includes a clear commit message to use.

(See WHATWG Working Mode: Changes for more details.)