Update "Launching the web share target" to use "create a fresh top-level traversable"

[alancutter]

Replace the last two steps of https://w3c.github.io/web-share-target/#launching-the-web-share-target

10. Let browsing context be the result of creating a new top-level browsing context.
11. Navigate browsing context to a new Request whose method is method, url is url, header list is header list, and body is body.

with a call to: https://whatpr.org/html/6315/document-sequences.html#create-a-fresh-top-level-traversable

[alancutter]

cc: @domenic