Cleanup: Pass std::string as const reference from components/

components/variations/service/variations_service_unittest.cc

Index: components/variations/service/variations_service_unittest.cc
diff --git a/components/variations/service/variations_service_unittest.cc b/components/variations/service/variations_service_unittest.cc
index 2d75afbc504947e0330aea2a9c47b5b89ad73de9..aeb5c199f69479eb565939f408b3f2936465a65f 100644
--- a/components/variations/service/variations_service_unittest.cc
+++ b/components/variations/service/variations_service_unittest.cc
@@ -60,7 +60,7 @@ class TestVariationsServiceClient : public VariationsServiceClient {
   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;
   }