Update .gn files to use new template syntax.

build/toolchain/gcc_toolchain.gni

 # 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.
 
 # This template defines a GCC toolchain.
 #
 # It requires the following variables specifying the executables to run:
 #  - cc
 #  - cxx
 #  - ar
 #  - ld
 # and the following which is used in the toolchain_args
 #  - toolchain_cpu_arch  (What "cpu_arch" should be set to when invoking a
 #                         build using this toolchain.)
 #  - toolchain_os  (What "os" should be set to when invoking a build using this
 #                   toolchain.)
 template("gcc_toolchain") {
   toolchain(target_name) {
+    assert(defined(invoker.cc), "gcc_toolchain() must specify a \"cc\" value")
+    assert(defined(invoker.cxx), "gcc_toolchain() must specify a \"cxx\" value")
+    assert(defined(invoker.ar), "gcc_toolchain() must specify a \"ar\" value")
+    assert(defined(invoker.ar), "gcc_toolchain() must specify a \"ld\" value")
+    assert(defined(invoker.toolchain_cpu_arch),
+                   "gcc_toolchain() must specify a \"toolchain_cpu_arch\"")
+    assert(defined(invoker.toolchain_os),
+                   "gcc_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.

[scottmg, 2014/03/27 18:01:00]

worth adding ${invoker.cc}? (or a bug for that)

+    cc = invoker.cc
+    cxx = invoker.cxx
+    ar = invoker.ar
+    ld = invoker.ld
+
     # Make these apply to all tools below.
     lib_prefix = "-l"
     lib_dir_prefix="-L"
 
     tool("cc") {
       # cflags_pch_c
       command = "$cc -MMD -MF \$out.d \$defines \$includes \$cflags \$cflags_c -c \$in -o \$out"
       description = "CC \$out"
       depfile = "\$out.d"
       deps = "gcc"
@@ skipping 25 matching lines @@
       description = "STAMP \$out"
     }
     tool("copy") {
       command = "ln -f \$in \$out 2>/dev/null || (rm -rf \$out && cp -af \$in \$out)"
       description = "COPY \$in \$out"
     }
 
     # When invoking this toolchain not as the default one, these args will be
     # passed to the build. They are ignored when this is the default toolchain.
     toolchain_args() {
-      cpu_arch = toolchain_cpu_arch
-      os = toolchain_os
+      cpu_arch = invoker.toolchain_cpu_arch
+      os = invoker.toolchain_os
     }
   }
 }