GN: Move towards only using / on Windows

tools/gn/source_file.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/source_file.h"
 
 #include "base/logging.h"
 #include "build/build_config.h"
 #include "tools/gn/filesystem_utils.h"
 #include "tools/gn/source_dir.h"
@@ skipping 35 matching lines @@
     return base::FilePath();
 
   std::string converted;
   if (is_system_absolute()) {
     if (value_.size() > 2 && value_[2] == ':') {
       // Windows path, strip the leading slash.
       converted.assign(&value_[1], value_.size() - 1);
     } else {
       converted.assign(value_);
     }
-    ConvertPathToSystem(&converted);
     return base::FilePath(UTF8ToFilePath(converted));
   }
 
   converted.assign(&value_[2], value_.size() - 2);
-  ConvertPathToSystem(&converted);
-  return source_root.Append(UTF8ToFilePath(converted));
+  return source_root.Append(UTF8ToFilePath(converted))
+      .NormalizePathSeparatorsTo('/');
 }