This patch contains a small optimization specifically for the case when `getDocument` is called with TypedArray-data. In that case we'll still hold onto that data, which could obviously be large, even after the "GetDocRequest"-message has been sent to the worker-thread.
In practice this will most likely not affect memory usage in any noticeable way, since the application calling `getDocument` will probably also be keeping a reference to the TypedArray-data. However, it seems like a good idea to ensure that the PDF.js API *itself* won't unnecessarily keep this data alive.
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