Add support for rendering / previewing PDFs in Qt WebEngine using PDFium.
PDFium is already used for rendering pages for printing on a QPrinter.
In the meantime, PDF.js can be used for viewing PDFs in the browser (with some limitations in e.g. file size of the pdf).
PDF.js needs some extra work by the developer (could be provided in an example) and is likely to use more resources.
Fully supporting this requires to implement the stubs for the Chromium extensions systems plus some extension APIs (mainly streams_private)