Update fetchpriority to match final spec

[pmeenan]

Summary

This updates the fetchpriority HTML attribute and priority fetch option to match the final spec. 1fetchprioritywas removed from the iFrame element andpriority` is no longer available on the Request object (it is part of the init config dictionary but that doesn't appear to have compat-level data).

Test results and supporting details

Here is the HTML spec PR and the Fetch spec PR.

[queengooborg]

Thanks, the links are LGTM! However, we set experimental status in BCD based upon browser implementation, not spec implementation -- because this feature set is only supported in Chrome, experimental should be set back to true.

[pmeenan]

@queengooborg Thanks. I assume by "browser" you mean the underlying engine, right? It has been enabled in all Chromium-based browsers for the better part of the last year.

I flipped it back to experimental and can come back and toggle it again when the Gecko or WebKit implementations reach a stable browser (both are in-flight).

[queengooborg]

Thanks for the quick changes, LGTM!