Update the styles for writing-mode, direction, text-orientation,
mix-blend-mode, backdrop-filter, and color-scheme.
Also, update the test, pseudo-get-computed-style.html, because it
doesn't really match the spec. For example, we update the pseudo-element
styles after we invoke the update callback, so the computed values of
these pseudo elements should be the default values inside the update callback.
Now the behavior of Gecko is the same as that of WebKit.
Differential Revision: https://phabricator.services.mozilla.com/D240406
584caf8dca