As of Chromium 63, ::shadow CSS selectors have been fully removed:
https://developers.google.com/web/updates/2017/10/remove-shadow-piercing
As a result, any usage of ::shadow in a stylesheet seems to get removed as soon
as the stylesheet is applied to an HTMLStyleElement. Since we have a couple of tests
that expect ::shadow selectors to be present, we must remove those expectations
since those selectors do not appear any longer.
Simply replace ::shadow with .editor to preserve the specificity of the
transformed rule. This isn't beautiful, but it's the only way to
guarantee that the styling transition is smooth enough to justify not
bumping the major.
