Also remove now-dead code that rustc warns about.
It turns out that we lost support for some of our custom URL schemes; I intend
to reimplement them, but I believe this will be significantly easier to do
once the legacy code is out of the way.
Implement Document.referrer
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1. More complete origin check in FileManagerThreadMsg
2. Add reference counting logic to file manage store and script API
3. Integrate the support of slicing
