Add the concept of a source set to GN.

tools/gn/output_file.h

 // 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.
 
 #ifndef TOOLS_GN_OUTPUT_FILE_H_
 #define TOOLS_GN_OUTPUT_FILE_H_
 
 #include <string>
 
 #include "tools/gn/build_settings.h"
@@ skipping 15 matching lines @@
   SourceFile GetSourceFile(const BuildSettings* build_settings) const {
     return SourceFile(build_settings->build_dir().value() + value_);
   }
 
   bool operator==(const OutputFile& other) const {
     return value_ == other.value_;
   }
   bool operator!=(const OutputFile& other) const {
     return value_ != other.value_;
   }
+  bool operator<(const OutputFile& other) const {
+    return value_ < other.value_;
+  }
 
  private:
   std::string value_;
 };
 
-#endif
+#endif  // TOOLS_GN_OUTPUT_FILE_H_