Support generic synchronization against an IPC::Channel

ipc/ipc_test.mojom

Index: ipc/ipc_test.mojom
diff --git a/ipc/ipc_test.mojom b/ipc/ipc_test.mojom
index 8c246d567665bc036a6442b564df3d8c8c24f449..06c0c2c477ac27346a3b65c176ecac6efe778f9c 100644
--- a/ipc/ipc_test.mojom
+++ b/ipc/ipc_test.mojom
@@ -4,6 +4,8 @@
 
 module IPC.mojom;
 
+import "mojo/common/event.mojom";
+
 interface SimpleTestDriver {
   ExpectValue(int32 value);
 
@@ -28,3 +30,19 @@ interface PingReceiver {
 interface IndirectTestDriver {
   GetPingReceiver(associated PingReceiver& request);
 };
+
+interface SynchronizationTestClient {
+  OnTokenReceived(string token) => ();
+};
+
+interface SynchronizationTestSender {
+  SendToken(SynchronizationTestReceiver receiver,
+            SynchronizationTestClient client);
+  Quit();
+};
+
+interface SynchronizationTestReceiver {
+  ReceiveToken(string token,
+               SynchronizationTestClient client,
+               mojo.common.mojom.Event& event);
+};