In bug 1967507 I changed the timing of AnonymousContent stylesheet
loading in a way that it perturbed a css cache test because of
accessiblecaret.css
https://hg.mozilla.org/mozilla-central/rev/a6a294ae1d18
However that made me realize that accessiblecaret.css is loaded
virtually in all processes, and it should be using the same mechanism we
use for UA sheets, rather than using all the CSS loader machinery
in-content. Same goes for details.css.
Expand GlobalStyleSheetCache to allow UA and Author sheets, and allow
ShadowRoot to get built-in stylesheets appended.
This allows accessiblecaret.css and details.css not to be marked as
content-accessible.
We could do the same at the document level for plaintext.css and co, but
that seems a bit less common, so maybe fine.
Differential Revision: https://phabricator.services.mozilla.com/D250909
e2ac38c2a8