Most CSP directives are ignored for PDF responses, but the
frame-ancestors directive should remain supported. Test that CSP can be
used to prevent embedding of a PDF file. Also test that a CSP can allow
embedding even if an X-Frame-Options: DENY directive is present.

Bug: 1107535
Change-Id: I3fbe5fca5e591bb670e0ad79babde191c83be0b2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2313999
Commit-Queue: Eric Lawrence [MSFT] <ericlaw@microsoft.com>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Cr-Commit-Position: refs/heads/master@{#791061}