GN: forward_variables_from shouldn't clobber vars.

build/toolchain/nacl_toolchain.gni

 # Copyright (c) 2014 The Native Client 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/toolchain/gcc_toolchain.gni")
 
 # This template defines a NaCl toolchain.
 #
 # It requires the following variables specifying the executables to run:
@@ skipping 20 matching lines @@
     } else {
       executable_extension = ".nexe"
     }
 
     forward_variables_from(invoker,
                            [
                              "ar",
                              "cc",
                              "cxx",
                              "deps",
-                             "is_clang",
-                             "is_nacl_glibc",
                              "ld",
                              "link_outputs",
                              "nm",
                              "readelf",
                              "strip",
                              "symbol_level",
                              "toolchain_cpu",
                            ])
 
+    if (defined(invoker.is_clang)) {
+      is_clang = invoker.is_clang
+    }
+    if (defined(invoker.is_nacl_glibc)) {
+      is_nacl_glibc = invoker.is_nacl_glibc
+    }
+
     # We do not support component builds or sanitizers with the NaCl toolchains.
     is_component_build = false
     clear_sanitizers = true
 
     # We do not support tcmalloc in the NaCl toolchains.
     use_allocator = "none"
 
     rebuild_define = "NACL_TC_REV=" + invoker.toolchain_revision
   }
 }