[OOPIF PDF] Create feature and flag for PDF OOPIF

Add PDF feature "PdfOopif" and flag "pdf-oopif". The intended use is to
be able to switch between using a MimeHandlerViewGuest or an OOPIF to
launch the PDF Viewer.

Bug: 1445746
Change-Id: Ifeaf2bcc66ecf6ac112aa46d97b6237fcd7e4f34
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4666784


Commit-Queue: Andy Phan <andyphan@chromium.org>
Reviewed-by: K. Moon <kmoon@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1171407}