GN: Get Mac build working for IPC target at least.

build/config/mac/mac_sdk.gni

+# Copyright 2014 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.
+
+declare_args() {
+  # Minimum supported version of the Mac SDK.
+  mac_sdk_min = "10.6"
+
+  # Path to a specific version of the Mac SDKJ, not including a backslash at
+  # the end. If empty, the path to the lowest version greater than or equal to
+  # mac_sdk_min is used.
+  mac_sdk_path = ""
+}
+
+if (is_chrome_branded && is_official_build) {
+  find_sdk_args = [ "--verify", mac_sdk_min "--sdk_path=" + mac_sdk_path ]
+} else {
+  find_sdk_args = [ mac_sdk_min ]
+}
+# Use "list lines" and take the first to get version string without a newline.
+find_sdk_lines =
+    exec_script("//build/mac/find_sdk.py", find_sdk_args, "list lines")
+mac_sdk_version = find_sdk_lines[0]
+if (mac_sdk_path == "") {
+  mac_sdk_root = "/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX"
+  mac_sdk_path = mac_sdk_root + mac_sdk_version + ".sdk"
+}