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
When constructing a Loader without passing a document, we added a DocGroup
parameter so that we could still use it to dispatch events to the DocGroup.
Delete NS_ENSURE_TRUE because new() is infallable.
Use another runnable pointer for calling dispatching because forget() will
nuke the pointer and we need to use evt afterwards.
MozReview-Commit-ID: Ce2K6j4pUhA
