These changes allow us to asynchronously load pre-loaded stylesheets, in a way
that's similar to ChromeUtils.compileScript. The new method returns a Promise
which resolves to the preloaded sheet once it's finished loading.
This will allow us to remove the last remaining use of synchronous channels in
moz-extension: URLs.
MozReview-Commit-ID: 7J52ff93YKT
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extra : rebase_source : 20fa013cdc7f5fbedb5ce671ede17765a2abbac2