Add the concept of a source set to GN.

tools/gn/output_file.h

Index: tools/gn/output_file.h
diff --git a/tools/gn/output_file.h b/tools/gn/output_file.h
index 9c5e4b2c205ae9168d30bb75925536a8d113042f..2e7a0ce1d76bfb6925b22396ce7a6fe791535336 100644
--- a/tools/gn/output_file.h
+++ b/tools/gn/output_file.h
@@ -33,9 +33,12 @@ class OutputFile {
   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_