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

chrome/browser/chromeos/settings/device_oauth2_token_service_unittest.cc

 // Copyright 2013 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 "chrome/browser/chromeos/settings/device_oauth2_token_service.h"
 
 #include "base/message_loop/message_loop.h"
 #include "base/prefs/testing_pref_service.h"
 #include "base/run_loop.h"
 #include "base/threading/sequenced_worker_pool.h"
@@ skipping 127 matching lines @@
   }
 
   // Utility method to set a value in Local State for the device refresh token
   // (it must have a non-empty value or it won't be used).
   void SetDeviceRefreshTokenInLocalState(const std::string& refresh_token) {
     scoped_testing_local_state_.Get()->SetUserPref(
         prefs::kDeviceRobotAnyApiRefreshToken,
         new base::StringValue(refresh_token));
   }
 
-  std::string GetValidTokenInfoResponse(const std::string email) {
+  std::string GetValidTokenInfoResponse(const std::string& email) {
     return "{ \"email\": \"" + email + "\","
            "  \"user_id\": \"1234567890\" }";
   }
 
   bool RefreshTokenIsAvailable() {
     return oauth2_service_->RefreshTokenIsAvailable(
         oauth2_service_->GetRobotAccountId());
   }
 
   std::string GetRefreshToken() {
@@ skipping 311 matching lines @@
   // Also make the robot account ID available. Verify that the token is
   // announced now.
   EXPECT_CALL(observer, OnRefreshTokenAvailable("robot@example.com"));
   SetRobotAccountId("robot@example.com");
   testing::Mock::VerifyAndClearExpectations(&observer);
 
   oauth2_service_->RemoveObserver(&observer);
 }
 
 }  // namespace chromeos