login: Add retrieve primary user to session manager dbus constants

Add retrieve primary user method name to public dbus constants so
it can be used by other packages.

BUG=b:77568272
TEST=cros_workon_make --board=nami system_api --test --install
compiles and builds.

Change-Id: Iaf17df366798df53a8be7544df7acc7cee969418
Signed-off-by: Mengqi Guo <mqg@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/1198047
Reviewed-by: Dan Erat <derat@chromium.org>
Reviewed-by: Mike Frysinger <vapier@chromium.org>
diff --git a/dbus/login_manager/dbus-constants.h b/dbus/login_manager/dbus-constants.h
index 0e402cd..d215003 100644
--- a/dbus/login_manager/dbus-constants.h
+++ b/dbus/login_manager/dbus-constants.h
@@ -37,6 +37,7 @@
     "ListStoredComponentPolicies";
 const char kSessionManagerRetrieveSessionState[] = "RetrieveSessionState";
 const char kSessionManagerRetrieveActiveSessions[] = "RetrieveActiveSessions";
+const char kSessionManagerRetrievePrimarySession[] = "RetrievePrimarySession";
 const char kSessionManagerStartTPMFirmwareUpdate[] = "StartTPMFirmwareUpdate";
 const char kSessionManagerStartDeviceWipe[] = "StartDeviceWipe";
 const char kSessionManagerHandleSupervisedUserCreationStarting[] =