Cleanup: Pass std::string as const reference if possible

media/cast/test/utility/input_builder.h

Index: media/cast/test/utility/input_builder.h
diff --git a/media/cast/test/utility/input_builder.h b/media/cast/test/utility/input_builder.h
index 4eb4ae52839d812300af36809d58ca1901183cc6..f8ec4d7e0289c0831777b4c0eafd4e292ac4924f 100644
--- a/media/cast/test/utility/input_builder.h
+++ b/media/cast/test/utility/input_builder.h
@@ -33,7 +33,7 @@ class InputBuilder {
   int GetIntInput() const;
 
  private:
-  bool ValidateInput(const std::string input) const;
+  bool ValidateInput(const std::string& input) const;
 
   const std::string title_;
   const std::string default_value_;