GN: forward_variables_from shouldn't clobber vars.

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.
 
 import("//build/config/nacl/config.gni")
 import("//build/config/sanitizers/sanitizers.gni")
 import("//build/toolchain/cc_wrapper.gni")
 import("//build/toolchain/goma.gni")
 import("//build/toolchain/toolchain.gni")
 
@@ skipping 394 matching lines @@
       current_cpu = invoker.toolchain_cpu
       current_os = invoker.toolchain_os
 
       # These values need to be passed through unchanged.
       host_toolchain = host_toolchain
       target_os = target_os
       target_cpu = target_cpu
 
       forward_variables_from(invoker,
                              [
-                               "is_clang",
-                               "is_component_build",
-                               "is_nacl_glibc",
                                "use_allocator",
                                "use_gold",
                                "symbol_level",
                              ])
 
+      if (defined(invoker.is_clang)) {
+        is_clang = invoker.is_clang

[agrieve, 2016/05/03 00:14:09]

why did these need to be broken out?

[brettw, 2016/05/03 01:02:44]

These are all global variables and in this case we specifically want to clobber
if they invoker ones are defined.

+      }
+      if (defined(invoker.is_component_build)) {
+        is_component_build = invoker.is_component_build
+      }
+      if (defined(invoker.is_nacl_glibc)) {
+        is_nacl_glibc = invoker.is_nacl_glibc
+      }
+
       if (defined(invoker.clear_sanitizers) && invoker.clear_sanitizers) {
         is_asan = false
         is_cfi = false
         is_lsan = false
         is_msan = false
         is_syzyasan = false
         is_tsan = false
         is_ubsan = false
         is_ubsan_vptr = false
       }
@@ skipping 35 matching lines @@
 
     forward_variables_from(invoker,
                            [
                              "toolchain_cpu",
                              "toolchain_os",
                              "use_gold",
                              "strip",
                            ])
   }
 }