vpn-manager: Accept PEM files

Try to open CA certificate files as PEM when trying to retrieve
subject information from certificates.

BUG=chromium:249363
TEST=Unit tests.

Change-Id: I41cb60ddcbb4e6a0e2eb1a4120a58d1f8955c344
Reviewed-on: https://gerrit.chromium.org/gerrit/60542
Reviewed-by: Christopher Wiley <wiley@chromium.org>
Reviewed-by: Darin Petkov <petkov@chromium.org>
Commit-Queue: Paul Stewart <pstew@chromium.org>
Tested-by: Paul Stewart <pstew@chromium.org>
1 file changed