GN: Fix compile errors with os==chromeos mojo/public build

build/config/BUILDCONFIG.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.
 
 # =============================================================================
 # BUILD FLAGS
 # =============================================================================
 #
 # This block lists input arguments to the build, along with their default
 # values. GN requires listing them explicitly so it can validate input and have
@@ skipping 46 matching lines @@
   is_asan = false
 
   # Compile for Leak Sanitizer to find leaks.
   is_lsan = false
 
   # Compile for Memory Sanitizer to find uninitialized reads.
   is_msan = false
 
   # Compile for Thread Sanitizer to find threading bugs.
   is_tsan = false
+
+  if (os == "chromeos") {
+    # Allows the target toolchain to be injected as arguments. This is needed
+    # to support the CrOS build system which supports per-build-configuration
+    # toolchains.
+    cros_use_custom_toolchain = false
+  }
 }
 
 # =============================================================================
 # OS DEFINITIONS
 # =============================================================================
 #
 # We set these various is_FOO booleans for convenience in writing OS-based
 # conditions.
 #
 # - is_android, is_chromeos, is_ios, and is_win should be obvious.
@@ skipping 412 matching lines @@
   }
   set_default_toolchain("//build/toolchain/android:$cpu_arch")
 } else if (is_linux) {
   if (is_clang) {
     host_toolchain = "//build/toolchain/linux:clang_$build_cpu_arch"
     set_default_toolchain("//build/toolchain/linux:clang_$cpu_arch")
   } else {
     host_toolchain = "//build/toolchain/linux:$build_cpu_arch"
     set_default_toolchain("//build/toolchain/linux:$cpu_arch")
   }
+  if (is_chromeos && cros_use_custom_toolchain) {
+    set_default_toolchain("//build/toolchain/cros:target")
+  }
 } else if (is_mac) {
   host_toolchain = "//build/toolchain/mac:clang"
   set_default_toolchain(host_toolchain)
 } else if (is_ios) {
   host_toolchain = "//build/toolchain/mac:host_clang"
   set_default_toolchain("//build/toolchain/mac:clang")
 }
 
 # ==============================================================================
 # COMPONENT SETUP
@@ skipping 164 matching lines @@
       if (defined(invoker.output_extension)) { output_extension = invoker.output_extension }
       if (defined(invoker.output_name)) { output_name = invoker.output_name }
       if (defined(invoker.public)) { public = invoker.public }
       if (defined(invoker.public_configs)) { public_configs = invoker.public_configs }
       if (defined(invoker.public_deps)) { public_deps = invoker.public_deps }
       if (defined(invoker.sources)) { sources = invoker.sources }
       if (defined(invoker.visibility)) { visibility = invoker.visibility }
     }
   }
 }