This allows popups and sidebars to use the chrome preferred
color-scheme.
This moves the responsibility of setting the content-preferred color
scheme to the appropriate browsers to the front-end (via tabs.css).
We still return the PreferredColorSchemeForContent() when there's no
pres context (e.g., for display:none in-process iframes). We could
potentially move a bunch of the pres-context data to the document
instead, but that should be acceptable IMO as for general web content
there's no behavior change in any case.
Differential Revision: https://phabricator.services.mozilla.com/D142578
