Mark the html:mark element to the new ROLE_MARK, which currently maps the same as role_text, and the xml-role of "mark", make sure we expose the roleDescription on Mac, and adjust the test so it tests that the attributes don't pick up any unexpected color for this particular element. So, the background attribute is empty when there is no unexpected, non-default background color.
Differential Revision: https://phabricator.services.mozilla.com/D52989
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extra : moz-landing-system : lando
