Add a flag to drop access to the session keyring

Now that chrome os is moving over to ext4 based directory encryption, the
encryption keys are stored in the session keyring.  Applications that don't need
to access the encrypted user data directory don't need access to this keyring.
Add a flag for applications to drop access to the session keyring when they
don't need it.

Bug: crbug.com/682419
TEST=autotest in a later CL

Change-Id: I3cb8f120d9f4891d9a13f7fe342b0388e9975605
Signed-off-by: Chirantan Ekbote <chirantan@google.com>
diff --git a/libminijail.h b/libminijail.h
index 14aeece..2bf3024 100644
--- a/libminijail.h
+++ b/libminijail.h
@@ -61,6 +61,8 @@
 void minijail_reset_signal_mask(struct minijail *j);
 void minijail_namespace_vfs(struct minijail *j);
 void minijail_namespace_enter_vfs(struct minijail *j, const char *ns_path);
+void minijail_new_session_keyring(struct minijail *j);
+
 /*
  * This option is *dangerous* as it negates most of the functionality of
  * minijail_namespace_vfs(). You very likely don't need this.