Remove obsolete OS handling in GN.

tools/gn/settings.cc

 // 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.
 
 #include "tools/gn/settings.h"
 
 #include "base/logging.h"
 #include "build/build_config.h"
 #include "tools/gn/filesystem_utils.h"
 
@@ skipping 11 matching lines @@
     toolchain_output_subdir_.value().append(output_subdir_name);
 
     DCHECK(!build_settings->build_dir().is_null());
     toolchain_output_dir_ = SourceDir(build_settings->build_dir().value() +
                                       toolchain_output_subdir_.value());
   }
   // The output dir will be null in some tests and when invoked to parsed
   // one-off data without doing generation.
   if (!toolchain_output_dir_.is_null())
     toolchain_gen_dir_ = SourceDir(toolchain_output_dir_.value() + "gen/");
-
-#if defined(OS_WIN)
-  target_os_ = WIN;
-#elif defined(OS_MACOSX)
-  target_os_ = MAC;
-#elif defined(OS_LINUX)
-  target_os_ = LINUX;
-#elif defined(OS_ANDROID)
-  // Currently we don't have an "Android" target OS, it looks just like Linux
-  // from our perspective.
-  target_os_ = LINUX;
-#else
-  #error implement me
-#endif
 }
 
 Settings::~Settings() {
 }