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

components/variations/service/variations_service_unittest.cc

 // Copyright (c) 2012 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 "components/variations/service/variations_service.h"
 
 #include <vector>
 
 #include "base/base64.h"
 #include "base/json/json_string_value_serializer.h"
@@ skipping 42 matching lines @@
   }
   bool OverridesRestrictParameter(std::string* parameter) override {
     if (restrict_parameter_.empty())
       return false;
     *parameter = restrict_parameter_;
     return true;
   }
   void OverrideUIString(uint32_t hash, const base::string16& string) override {}
   void OnInitialStartup() override {}
 
-  void set_restrict_parameter(std::string value) {
+  void set_restrict_parameter(const std::string& value) {
     restrict_parameter_ = value;
   }
 
  private:
   std::string restrict_parameter_;
 
   DISALLOW_COPY_AND_ASSIGN(TestVariationsServiceClient);
 };
 
 // A test class used to validate expected functionality in VariationsService.
@@ skipping 461 matching lines @@
         << test.pref_value_before << ", " << test.version << ", "
         << test.latest_country_code;
 
     histogram_tester.ExpectUniqueSample(
         "Variations.LoadPermanentConsistencyCountryResult",
         test.expected_result, 1);
   }
 }
 
 }  // namespace variations