R1:fb4ce851c4ae

Add a PDF document "rendering" engine

Summary:
Depends on D19251. Ref T13105. This adds rendering engine support for PDFs.

It doesn't actually render them, it just renders a link which you can click to view them in a new window. This is much easier than actually rendering them inline and at least 95% as good most of the time (and probably more-than-100%-as-good some of the time).

This makes PDF a viewable MIME type by default and adds a narrow CSP exception for it. See also T13112.

Test Plan…