Add width and height attributes to source elements

[yoavweiss]

This addresses #4968
I believe this has multi-implementer interest, based on #4968 (comment), but will let others comment to that effect.

This doesn't yet have tests, as I wanted to first verify the approach I took makes sense. I'll make sure tests are added before landing. Similarly, I'll make sure implementation bugs are opened.

/cc @zcorpan @annevk @domenic @chrishtr @emilio @jensimmons @cbiesinger

• At least two implementers are interested (and none opposed):
  • Chromium
  • Firefox: in this thread and mapping a <source>'s width/height attributes to width/height/aspect-ratio CSS properties on the <img> in a <picture> mozilla/standards-positions#485
• Tests are written and can be reviewed and commented upon at:
  • Split the big test() up into multiple small ones web-platform-tests/wpt#27745
• Implementation bugs are filed:
  • Chrome: https://bugs.chromium.org/p/chromium/issues/detail?id=1137794
  • Firefox: https://bugzilla.mozilla.org/show_bug.cgi?id=1694741
  • Safari: https://bugs.webkit.org/show_bug.cgi?id=222368

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

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