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

components/metrics/cloned_install_detector.h

 // Copyright 2014 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 COMPONENTS_METRICS_CLONED_INSTALL_DETECTOR_H_
 #define COMPONENTS_METRICS_CLONED_INSTALL_DETECTOR_H_
 
 #include "base/gtest_prod_util.h"
 #include "base/memory/ref_counted.h"
 #include "base/memory/weak_ptr.h"
@@ skipping 28 matching lines @@
 
   static void RegisterPrefs(PrefRegistrySimple* registry);
 
  private:
   FRIEND_TEST_ALL_PREFIXES(ClonedInstallDetectorTest, SaveId);
   FRIEND_TEST_ALL_PREFIXES(ClonedInstallDetectorTest, DetectClone);
 
   // Converts raw_id into a 24-bit hash and stores the hash in |local_state|.
   // |raw_id| is not a const ref because it's passed from a cross-thread post
   // task.
-  void SaveMachineId(PrefService* local_state, std::string raw_id);
+  void SaveMachineId(PrefService* local_state, const std::string& raw_id);
 
   scoped_refptr<MachineIdProvider> raw_id_provider_;
   base::WeakPtrFactory<ClonedInstallDetector> weak_ptr_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(ClonedInstallDetector);
 };
 
 }  // namespace metrics
 
 #endif  // COMPONENTS_METRICS_CLONED_INSTALL_DETECTOR_H_