Read a media element's loading attribute after awaiting a stable state

[zcorpan]

The parser appends attributes one at a time, so <video src loading=lazy> reached the resource
selection algorithm with loading still absent, was treated as Eager, and delayed the load event
until the video was scrolled into view.

Fixes #12854.

• At least two implementers are interested (and none opposed):
  • Bug fix to the recently added media loading attribute (Add support for video and audio element lazy-loading via the loading attribute #11980); Chromium already behaves this way.
• Tests are written and can be reviewed and commented upon at:
  • Test that loading=lazy videos don't delay load in either attribute order web-platform-tests/wpt#62828
• Implementation bugs are filed:
  • Chromium: not needed, Chrome Canary passes the test
  • Gecko: not needed, loading on media elements isn't implemented yet
  • WebKit: not needed, loading on media elements isn't implemented yet
• Corresponding HTML AAM & ARIA in HTML issues & PRs: n/a
• MDN issue is filed: n/a
• The top of this comment includes a clear commit message to use.

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