Change subframe traverse navigate event timing

[domenic]

This fires navigate events for subframes during a traversal earlier, before the unload event and in parallel with any fetches that occur. This is more consistent with non-traverse navigate events, and with top-level traverse navigate events.

WICG/navigation-api#207 has some previous relevant discussion. This represents moving from that issue's "late" navigate events to "early-but-still-async". It turns out the complexity was not too bad, when we got around to it in Chromium, and was actually helpful in order to fix a bug where traverse navigate events weren't firing when pages were bfcached.

• At least two implementers are interested (and none opposed):
  • I would like to count this as a bug fix against the navigation API that does not require explicit additional interest, since other navigation API implementations have barely started. Also in navigate event timing on history traversals WICG/navigation-api#207 we had both @annevk and @smaug---- involved with "We discussed this offline and didn't come to any firm conclusions; everyone was pretty flexible."
• Tests are written and can be reviewed and commented upon at:
  • Fire the navigate event earlier for cross-document traversals web-platform-tests/wpt#43008
• Implementation bugs are filed:
  • Chromium: https://bugs.chromium.org/p/chromium/issues/detail?id=1475907
  • Gecko: assuming not needed because navigation API implementation has not started
  • WebKit: assuming not needed because navigation API implementation has just started and hasn't implemented this part yet
  • Deno (only for timers, structured clone, base64 utils, channel messaging, module resolution, web workers, and web storage): N/A
  • Node.js (only for timers, structured clone, base64 utils, channel messaging, and module resolution): N/A
• MDN issue is filed: N/A, too detailed
• The top of this comment includes a clear commit message to use.

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

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/browsing-the-web.html ( diff )

[domfarolino]

This change looks good, thanks for the context in the OP too.