Set requests' render-blocking flag when fetching resources for render-blocking elements

[xiaochengh]

The Fetch-side counterpart is whatwg/fetch#1432.

This is also a minor follow-up of #7474.

• At least two implementers are interested (and none opposed):
  • See Add the render-blocking mechanism and blocking=render attribute #7474
• Tests are written and can be reviewed and commented upon at:
  • Fetch request's render-blocking flag is not web-exposed and can't be tested against with WPT
  • The flag may affect priority, which is a UA-defined object and cannot be tested against with WPT, either.
    • Blink-specific tests are at https://chromium-review.googlesource.com/c/chromium/src/+/3627294
• Implementation bugs are filed:
  • See Add the render-blocking mechanism and blocking=render attribute #7474

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

---

/infrastructure.html ( diff )
/links.html ( diff )
/scripting.html ( diff )
/semantics.html ( diff )
/webappapis.html ( diff )

[xiaochengh]

@domenic PTAL

[xiaochengh]

Thanks for reviewing!

[domenic]

This LGTM. Tagging "do not merge yet" until the Fetch PR is landed.

I will be OOO next week. @annevk, if the Fetch PR lands while I am out, it'd be good to land this too :)

[domenic]

A few nits before we can merge

[xiaochengh]

Thanks for reviewing! Updated.