Fifo order should be preserved for messages on associated interfaces.

mojo/public/interfaces/bindings/tests/test_associated_interfaces.mojom

Index: mojo/public/interfaces/bindings/tests/test_associated_interfaces.mojom
diff --git a/mojo/public/interfaces/bindings/tests/test_associated_interfaces.mojom b/mojo/public/interfaces/bindings/tests/test_associated_interfaces.mojom
index 3858330aecf242a6675b19d7fd9dd6d23a582e50..6df1e8be3ecaac8b1de1be467daaf887e99066ea 100644
--- a/mojo/public/interfaces/bindings/tests/test_associated_interfaces.mojom
+++ b/mojo/public/interfaces/bindings/tests/test_associated_interfaces.mojom
@@ -40,6 +40,11 @@ interface IntegerSender {
   Send(int32 value);
 };
 
+interface StringSender {
+  Echo(string value) => (string value);
+  Send(string value);
+};
+
 interface IntegerSenderConnection {
   GetSender(associated IntegerSender& sender);
   AsyncGetSender() => (associated IntegerSender sender);
@@ -50,6 +55,11 @@ interface IntegerSenderConnectionAtBothEnds {
   SetSender(associated IntegerSender sender) => (int32 value);
 };
 
+interface SenderConnection {
+  GetIntegerSender(associated IntegerSender& sender);
+  GetStringSender(associated StringSender& sender);
+};
+
 interface AssociatedPingProvider {
   GetPing(associated PingService& request);
 };