Fix the //sandbox/mac build when using a modern (10.7+) SDK.

sandbox/mac/BUILD.gn

 # 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.
 
 component("sandbox") {
   sources = [
     "bootstrap_sandbox.cc",
     "bootstrap_sandbox.h",
     "dispatch_source_mach.cc",
     "dispatch_source_mach.h",
@@ skipping 10 matching lines @@
     "xpc.h",
     "xpc_message_server.cc",
     "xpc_message_server.h",
   ]
 
   defines = [ "SANDBOX_IMPLEMENTATION" ]
   libs = [ "bsm" ]
 
   deps = [
     "//base",
-    ":generate_stubs",
   ]
+
+  # When the build SDK is 10.6, generate a dynamic stub loader. When the
+  # SDK is higher, then libxpc.dylib will be loaded automatically as part
+  # of libSystem, and only forward declarations of private symbols are
+  # necessary.
+  if (mac_sdk_version == "10.6") {
+    deps += [ ":generate_stubs" ]
+  }
 }
 
 generate_stubs_script = "//tools/generate_stubs/generate_stubs.py"
 generate_stubs_header = "xpc_stubs_header.fragment"
 generate_stubs_sig_public = "xpc_stubs.sig"
 generate_stubs_sig_private = "xpc_private_stubs.sig"
 generate_stubs_project = "sandbox/mac"
 generate_stubs_output_stem = "xpc_stubs"
 
 action("generate_stubs") {
@@ skipping 29 matching lines @@
     "Foundation.framework",
   ]
 
   deps = [
     ":sandbox",
     "//base",
     "//base/test:run_all_unittests",
     "//testing/gtest",
   ]
 }