Add SurfaceReference class.

cc/ipc/traits_test_service.mojom

Index: cc/ipc/traits_test_service.mojom
diff --git a/cc/ipc/traits_test_service.mojom b/cc/ipc/traits_test_service.mojom
index b94ce2ff79efccda63eb23656453058f0dfe0f49..7948f121261cd587dc2a7013316f9d65425bf58e 100644
--- a/cc/ipc/traits_test_service.mojom
+++ b/cc/ipc/traits_test_service.mojom
@@ -16,6 +16,7 @@ import "cc/ipc/returned_resource.mojom";
 import "cc/ipc/selection.mojom";
 import "cc/ipc/shared_quad_state.mojom";
 import "cc/ipc/surface_id.mojom";
+import "cc/ipc/surface_reference.mojom";
 import "cc/ipc/surface_sequence.mojom";
 import "cc/ipc/transferable_resource.mojom";
 
@@ -57,6 +58,9 @@ interface TraitsTestService {
   EchoSurfaceId(SurfaceId s) => (SurfaceId pass);
 
   [Sync]
+  EchoSurfaceReference(SurfaceReference r) => (SurfaceReference pass);
+
+  [Sync]
   EchoSurfaceSequence(SurfaceSequence s) => (SurfaceSequence pass);
 
   [Sync]