Allow DBSC flags on Mac and Linux

For people looking to test the DBSC protocol, it's nice to allow testing
from diverse
clients. #enable-bound-session-credentials-software-keys-for-manual-testing
allows use of software keys for testing on Mac, Win, and Linux, so we do
the same for standard DBSC.

Change-Id: I692c29bb084f72252cf2329916d471fd846621e9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6286806
Reviewed-by: thefrog <thefrog@chromium.org>
Commit-Queue: thefrog <thefrog@chromium.org>
Commit-Queue: Daniel Rubery <drubery@chromium.org>
Auto-Submit: Daniel Rubery <drubery@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1422702}