Fix the Mac toolchain definition in the GN build.

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")
 
@@ skipping 41 matching lines @@
 # 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 ], "value")
 
 # Shared toolchain definition. Invocations should set toolchain_os to set the
 # build args in this definition.
 template("mac_clang_toolchain") {
   toolchain(target_name) {
+    assert(defined(invoker.cc),
+           "mac_clang_toolchain() must specify a \"cc\" value")
+    assert(defined(invoker.cxx),
+           "mac_clang_toolchain() must specify a \"cxx\" value")
+    assert(defined(invoker.ld),
+           "mac_clang_toolchain() must specify a \"ld\" value")
+    assert(defined(invoker.toolchain_os),
+           "mac_clang_toolchain() must specify a \"toolchain_os\"")
+
+    # We can't do string interpolation ($ in strings) on things with dots in
+    # them. To allow us to use $cc below, for example, we create copies of
+    # these values in our scope.
+    cc = invoker.cc
+    cxx = invoker.cxx
+    ld = invoker.ld
+
     # Make these apply to all tools below.
     lib_prefix = "-l"
     lib_dir_prefix="-L"
 
     tool("cc") {
       command = "$cc -MMD -MF \$out.d \$defines \$includes \$cflags \$cflags_c \$cflags_pch_c -c \$in -o \$out"
       description = "CC \$out"
       depfile = "\$out.d"
       deps = "gcc"
     }
@@ skipping 45 matching lines @@
     tool("stamp") {
       command = "\${postbuilds}touch \$out"
       description = "STAMP \$out"
     }
     tool("copy") {
       command = "ln -f \$in \$out 2>/dev/null || (rm -rf \$out && cp -af \$in \$out)"
       description = "COPY \$in \$out"
     }
 
     toolchain_args() {
-      os = toolchain_os
+      os = invoker.toolchain_os
     }
   }
 }
 
 # 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"
 }