Add a instance groups to Connect(), and introduce an Identity struct.

mojo/shell/tests/connect/connect_test.mojom

Index: mojo/shell/tests/connect/connect_test.mojom
diff --git a/mojo/shell/tests/connect/connect_test.mojom b/mojo/shell/tests/connect/connect_test.mojom
index 356061e466d235846789b7f3b93dc8035b3ac794..3eeaa126a9af233e4f31909212dd80287892dc7d 100644
--- a/mojo/shell/tests/connect/connect_test.mojom
+++ b/mojo/shell/tests/connect/connect_test.mojom
@@ -14,6 +14,8 @@ interface StandaloneApp {
   // the standalone app's CapabilityFilter, but whose enclosing package is not.
   // The connection should be blocked and title should be "uninitialized".
   ConnectToAllowedAppInBlockedPackage() => (string title);
+
+  GetInstance() => (string instance);
 };
 
 struct ConnectionState {