Set up mac v2 sandbox policy for pdf compositor

Pdf compositor service composites contents printed from different
renderer process into a complete pdf document. This is needed for
general printing when site isolation is enabled, because a web page with
out of process iframes may be rendered in multiple renderers.

Therefore, pdf compositor service needs to access font access.
This CL set up pdf compositor service's own seatbelt policy.

BUG=809763, 455764

Change-Id: I4a78ef793c1ea5e52b167a74fd02091f197bd61f
Reviewed-on: https://chromium-review.googlesource.com/912316
Reviewed-by: Alex Moshchuk <alexmos@chromium.org>
Reviewed-by: Greg Kerr <kerrnel@chromium.org>
Reviewed-by: Robert Sesek <rsesek@chromium.org>
Commit-Queue: Wei Li <weili@chromium.org>
Cr-Commit-Position: refs/heads/master@{#536153}
3 files changed