Add the SeatbeltExec classes to facilitate the V2 sandbox.

sandbox/mac/seatbelt.proto

Index: sandbox/mac/seatbelt.proto
diff --git a/sandbox/mac/seatbelt.proto b/sandbox/mac/seatbelt.proto
new file mode 100644
index 0000000000000000000000000000000000000000..4615402053c473fa64d6bf6048698aecbe237562
--- /dev/null
+++ b/sandbox/mac/seatbelt.proto
@@ -0,0 +1,17 @@
+// Copyright 2017 The Chromium OS Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+syntax = "proto2";
+option optimize_for = LITE_RUNTIME;
+
+package sandbox.mac;
+
+// Optional fields are considered best practice, but since the client and server
+// come from the same code base, this uses required fields to save on
+// validation.
+
+message SandboxPolicy {
+  required string profile = 1;
+  map<string, string> params = 2;
+}