Guard the policy added flags with sentinel flags.
Guard the policy added flags with sentinel flags.

That way it will be easier for the client to decide and do filtering
on those flags whenever required.

BUG=chromium:221354
TEST=Set the StartUpFlags policy and verify that the sentinel flags are present.

Change-Id: I3ad56f0c6b92eb9a2bc66783d8712da2a3c47d57
Reviewed-on: https://gerrit.chromium.org/gerrit/46162
Tested-by: Julian Pastarmov <c4978c99f3b8ea0c0f0653fba7cc0f598e925751@chromium.org>
Reviewed-by: Chris Masone <98e3d65c6b0e9e2391eb4c2e8b1e572b77e544ed@chromium.org>
Commit-Queue: Julian Pastarmov <c4978c99f3b8ea0c0f0653fba7cc0f598e925751@chromium.org>

chadversary/chromiumos.platform.login_manager,chadversary/chromiumos.platform.login_manager,chadversary/chromiumos.platform.login_manager