Mozilla bug 1618295 - Add imagesrcset and imagesizes attribute to the HTML parser; r=hsivonen

See https://html.spec.whatwg.org/multipage/semantics.html#the-link-element:attr-link-imagesrcset.

Depends on D71949

Differential Revision: https://phabricator.services.mozilla.com/D72083

Author: EdgarChen