GN: Move towards only using / on Windows

tools/gn/source_dir.cc

Index: tools/gn/source_dir.cc
diff --git a/tools/gn/source_dir.cc b/tools/gn/source_dir.cc
index 81340f94d2b4988af701bdfbae076310e4b571f8..c59cf88cd61136bc4e1fa6db7620b8c55e736736 100644
--- a/tools/gn/source_dir.cc
+++ b/tools/gn/source_dir.cc
@@ -130,14 +130,13 @@ base::FilePath SourceDir::Resolve(const base::FilePath& source_root) const {
     } else {
       converted.assign(value_);
     }
-    ConvertPathToSystem(&converted);
     return base::FilePath(UTF8ToFilePath(converted));
   }
 
   // String the double-leading slash for source-relative paths.
   converted.assign(&value_[2], value_.size() - 2);
-  ConvertPathToSystem(&converted);
-  return source_root.Append(UTF8ToFilePath(converted));
+  return source_root.Append(UTF8ToFilePath(converted))
+      .NormalizePathSeparatorsTo('/');
 }
 
 void SourceDir::SwapValue(std::string* v) {