Add/remove surface references via MojoCompositorFrameSink.

cc/ipc/surface_reference.mojom

Index: cc/ipc/surface_reference.mojom
diff --git a/cc/ipc/surface_reference.mojom b/cc/ipc/surface_reference.mojom
new file mode 100644
index 0000000000000000000000000000000000000000..ecdd7ed485c4d8058cee44fc52e2299dcd0b65fa
--- /dev/null
+++ b/cc/ipc/surface_reference.mojom
@@ -0,0 +1,12 @@
+// Copyright 2016 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+module cc.mojom;
+
+import "cc/ipc/surface_id.mojom";
+
+struct SurfaceReference {
+  SurfaceId parent_id;
+  SurfaceId child_id;
+};