Make use_goma=true also work for Mac GN.

build/toolchain/mac/BUILD.gn

 # Copyright (c) 2013 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.
 
 # TODO(brettw) Use "gcc_toolchain.gni" like the Linux toolchains. This requires
 # some enhancements since the commands on Mac are slightly different than on
 # Linux.
 
 import("../goma.gni")
 
 # Should only be running on Mac.
 assert(is_mac || is_ios)
 
 import("//build/toolchain/clang.gni")
 import("//build/toolchain/goma.gni")
 
+if (use_goma) {
+  goma_prefix = "$goma_dir/gomacc "
+} else {
+  goma_prefix = ""
+}
+
 if (is_clang) {
   cc = rebase_path("//third_party/llvm-build/Release+Asserts/bin/clang",
                    root_build_dir)
   cxx = rebase_path("//third_party/llvm-build/Release+Asserts/bin/clang++",
                     root_build_dir)
 } else {
   cc = "gcc"
   cxx = "g++"
 }
+cc = goma_prefix + cc
+cxx = goma_prefix + cxx
 ld = cxx
 
 # This will copy the gyp-mac-tool to the build directory. We pass in the source
 # file of the win tool.
 gyp_mac_tool_source =
     rebase_path("//tools/gyp/pylib/gyp/mac_tool.py", root_build_dir)
 exec_script("setup_toolchain.py", [ gyp_mac_tool_source ])
 
 # Shared toolchain definition. Invocations should set toolchain_os to set the
 # build args in this definition.
@@ skipping 158 matching lines @@
 # Toolchain representing the target build (either mac or iOS).
 mac_clang_toolchain("clang") {
   toolchain_os = os
 }
 
 # This toolchain provides a way for iOS target compiles to reference targets
 # compiled for the host system. It just overrides the OS back to "mac".
 mac_clang_toolchain("host_clang") {
   toolchain_os = "mac"
 }