Fix compilation with Xcode version of clang.

build/toolchain/toolchain.gni

 # Copyright 2015 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.
 
 # Toolchain-related configuration that may be needed outside the context of the
 # toolchain() rules themselves.
 
 import("//build/config/chrome_build.gni")
 
 declare_args() {
@@ skipping 31 matching lines @@
     clang_version = "3.9.0"
 
     if (llvm_force_head_revision) {
       # TODO(hans): Remove after next Clang roll.
       # ToT Clang has a higher version number.
       clang_version = "4.0.0"
     }
   }
 }
 
-assert(!use_xcode_clang || is_ios,
+# Check target_os here instead of is_ios as this file is loaded for secondary
+# toolchain (host toolchain in particular) but the argument is the same for
+# all toolchains.
+assert(!use_xcode_clang || target_os == "ios",
        "Using Xcode's clang is only supported in iOS builds")
 
 # Subdirectory within root_out_dir for shared library files.
 # TODO(agrieve): GYP sets this to "lib" for Linux & Android, but this won't work
 #     in GN until support for loadable_module() is added.
 #     See: https://codereview.chromium.org/1236503002/
 shlib_subdir = "."
 
 # Root out dir for shared library files.
 root_shlib_dir = root_out_dir
@@ skipping 32 matching lines @@
 stamp_description = "STAMP {{output}}"
 copy_description = "COPY {{source}} {{output}}"
 if (host_os == "win") {
   stamp_command = "$python_path gyp-win-tool stamp {{output}}"
   copy_command =
       "$python_path gyp-win-tool recursive-mirror {{source}} {{output}}"
 } else {
   stamp_command = "touch {{output}}"
   copy_command = "ln -f {{source}} {{output}} 2>/dev/null || (rm -rf {{output}} && cp -af {{source}} {{output}})"
 }