Fix a stray reference to 'build_os' in BUILD.gn.

BUILD.gn

 # Copyright 2014 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/android/config.gni")
 import("//build/config/arm.gni")
 import("//build/config/mips.gni")
 
 # Because standalone V8 builds are not supported, assume this is part of a
 # Chromium build.
@@ skipping 1378 matching lines @@
     sources += [ "src/base/platform/platform-posix.cc" ]
   }
 
   if (is_linux) {
     sources += [ "src/base/platform/platform-linux.cc" ]
 
     libs = [ "rt" ]
   } else if (is_android) {
     defines += [ "CAN_USE_VFP_INSTRUCTIONS" ]
 
-    if (build_os == "mac") {
+    if (host_os == "mac") {
       if (current_toolchain == host_toolchain) {
         sources += [ "src/base/platform/platform-macos.cc" ]
       } else {
         sources += [ "src/base/platform/platform-linux.cc" ]
       }
     } else {
       sources += [ "src/base/platform/platform-linux.cc" ]
       if (current_toolchain == host_toolchain) {
         defines += [ "V8_LIBRT_NOT_AVAILABLE" ]
       }
@@ skipping 180 matching lines @@
       sources += [
         "src/d8-debug.cc",
         "$target_gen_dir/d8-js.cc",
       ]
     }
     if (v8_enable_i18n_support) {
       deps += [ "//third_party/icu" ]
     }
   }
 }