The `BaseCMapReaderFactory`, `BaseStandardFontDataFactory`, and `BaseWasmFactory` classes are all very similar, and the only difference is really in their respective `fetch` methods.
By have the worker-thread "compute" the complete `filename` it's possible to simplify the `BaseCMapReaderFactory.prototype.fetch` method, which will allow future improvements to all of these classes.
A couple of things to note:
- This code is unused, and it's not even bundled, in the Firefox PDF Viewer.
- In browsers it's unused by default, and worker-thread fetching will always be used when possible since that's more efficient.
*Please note:* For users that provide a custom `CMapReaderFactory` instance when calling `getDocument` this could be a breaking change, however it's unlikely that any such users exist.
(The *internal* format of this data was changed previously in PR 18951, and there hasn't been a single question/complaint about it in well over a year.)
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