Make use_goma=true also work for Mac GN.

Since toolchain/mac/BUILD.gn did not have anything to support
use_goma, even if use_goma=true set, the flags was just ignored.

BUG=459435

Review URL: https://codereview.chromium.org/932013004

Cr-Original-Commit-Position: refs/heads/master@{#316758}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 874b8fe5991b4f57088f3ff4218cc8e8acd2b18d

toolchain/mac/BUILD.gn

@@ -14,6 +14,12 @@ 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)
@@ -23,6 +29,8 @@ if (is_clang) {
   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