GN: Move towards only using / on Windows

tools/gn/setup.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/setup.h"
 
 #include <stdlib.h>
 
 #include <algorithm>
 
@@ skipping 276 matching lines @@
   }
 
   if (scheduler_.verbose_logging())
     scheduler_.Log("Using source root", FilePathToUTF8(root_path));
   build_settings_.SetRootPath(root_path);
 
   return true;
 }
 
 bool Setup::FillBuildDir(const std::string& build_dir) {
-  std::string normalized_build_dir = PathToSystem(build_dir);
-
   SourceDir resolved =
       SourceDirForCurrentDirectory(build_settings_.root_path()).
-          ResolveRelativeDir(normalized_build_dir);
+          ResolveRelativeDir(build_dir);
   if (resolved.is_null()) {
     Err(Location(), "Couldn't resolve build directory.",
         "The build directory supplied (\"" + build_dir + "\") was not valid.").
         PrintToStdout();
     return false;
   }
 
   if (scheduler_.verbose_logging())
     scheduler_.Log("Using build dir", resolved.value());
   build_settings_.SetBuildDir(resolved);
@@ skipping 121 matching lines @@
 }
 
 void DependentSetup::RunPreMessageLoop() {
   CommonSetup::RunPreMessageLoop();
 }
 
 bool DependentSetup::RunPostMessageLoop() {
   return CommonSetup::RunPostMessageLoop();
 }