Make sure use_xcode_clang is only used in iOS builds.

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 15 matching lines @@
     # Clang compiler version. Clang files are placed at version-dependent paths.
     clang_version = "3.9.0"
   }
 
   # Compile with Xcode version of clang instead of hermetic version shipped
   # with the build. Used on iOS to ship official builds (as they are built
   # with the version of clang shipped with Xcode).
   use_xcode_clang = is_ios && is_official_build
 }
 
+assert(!use_xcode_clang || is_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
 if (shlib_subdir != ".") {
   root_shlib_dir += "/$shlib_subdir"
@@ skipping 30 matching lines @@
 stamp_description = "STAMP {{output}}"
 copy_description = "COPY {{source}} {{output}}"
 if (host_os == "win") {
   stamp_command = "$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}})"
 }