Bypass blob URL partitioning for top-level self fetches

[recvfrom]

Implements the fetch spec side of bypassing partitioning when a top-level blob URL document attempts to fetch itself. For more info see: w3c/FileAPI#210

• At least two implementers are interested (and none opposed):
  • Implemented in Chromium: https://crbug.com/426787402
  • Possibly support from Safari: Top-level navigations to blob URLs with media mime types result in resource load subject to partitioning w3c/FileAPI#210 (comment)
• Tests are written and can be reviewed and commented upon at:
  • https://chromium-review.googlesource.com/c/chromium/src/+/7160405
• Implementation bugs are filed:
  • Chromium: https://crbug.com/426787402
  • Gecko: https://bugzilla.mozilla.org/show_bug.cgi?id=2000324
  • WebKit: https://bugs.webkit.org/show_bug.cgi?id=302558
  • Deno (not for CORS changes): N/A since Deno does not partition storage or more generally implement the same-origin policy: https://docs.deno.com/runtime/reference/web_platform_apis/#spec-deviations
• MDN issue is filed: TODO
• The top of this comment includes a clear commit message to use.

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

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[annevk]

I wonder if there are cases where this would be null and you can get this far into a fetch. Hmm.

[domfarolino]

Specifically where client is non-null but not associated with a Window object? Would worker fetches count here? Maybe even navigation requests themselves, with their reserved client machinery...

[recvfrom]

I mirrored this off of the logic in https://fetch.spec.whatwg.org/#populate-request-from-client, but I'm not certain that the cases there are possible here as well...