Add the SeatbeltExec classes to facilitate the V2 sandbox.

sandbox/mac/seatbelt.proto

+// 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 SandboxParams {
+  required string policy = 1;
+  map<string, string> params = 2;
+}