depot_tools: add mac_toolchain shim

This tool will soon be added as a gclient hook for Mac developers to install the
requested Xcode version on their workstations, and it's easier if gclient can
assume it on PATH.

BUG=475693
R=iannucci@chromium.org

Change-Id: I6af90755bb2ee6de1e19e3a475d9a23bf75888c3
Reviewed-on: https://chromium-review.googlesource.com/756888Reviewed-by: Robbie Iannucci <iannucci@chromium.org>
Reviewed-by: Aaron Gable <agable@chromium.org>
Commit-Queue: Sergey Berezin <sergeyberezin@chromium.org>

cipd_manifest.txt

@@ -12,3 +12,6 @@ infra/tools/luci/vpython/${platform}  git_revision:a3d636052088db3daa48413b3e209
 
 # LUCI editor
 infra/tools/luci/led/${platform}  git_revision:d1e9a52e1c8414cd0a0df8bec6c37f325e0fbaa1
+
+# Mac toolchain installer
+infra/tools/mac_toolchain/${os=mac}-${arch} git_revision:7eb9e7d8efe8bfec749e364470c5e7a7260c9ac2

mac_toolchain

+#!/bin/bash
+
+# Copyright 2017 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.
+
+MYPATH=$(dirname "${BASH_SOURCE[0]}")
+
+source "$MYPATH/cipd_bin_setup.sh"
+cipd_bin_setup &> /dev/null
+
+exec "$MYPATH/.cipd_bin/mac_toolchain" "$@"