Add a hook for reporting file download ended (complete or canceled) and provide the downloaded file path there. Required for WebDriver BiDibrowsingContext.downloadEnd event.

Refactoring

In order to provide the proper events' order, the "WebDriver BiDi download started" hook was moved into "handle as a download".
TODO: check if the<var>navigable</var> and<var>navigationId</var> really can be null there.

Specification

As long as the download processing is non-normative, the generic description was used:

• "If the file downloading was canceled by user or by the user agent"
• "If the file downloading faced any problems"
• "When the file downloading successfully completed"

• At least two implementers are interested (and none opposed):

• Chromium
• Mozilla

• Tests are written and can be reviewed and commented upon at:
  • [wdspec]browsingContext.downloadEnd event web-platform-tests/wpt#52932
• Implementation bugs are filed:
  • WebDriver BiDi:browsingContext.downloadEnd event w3c/webdriver-bidi#923
  • Chromium:feat: implementbrowsingContext.downloadFinished event GoogleChromeLabs/chromium-bidi#3427
  • Gecko:https://bugzilla.mozilla.org/show_bug.cgi?id=1970293
  • WebKit: N/A
  • 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
• CorrespondingHTML AAM &ARIA in HTML issues & PRs: N/A
• MDN issue is filed: N/A
• The top of this comment includes aclear commit message to use.

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