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

components/gcm_driver/gcm_account_tracker_unittest.cc

 // 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.
 
 #include "components/gcm_driver/gcm_account_tracker.h"
 
 #include <map>
 #include <string>
 
 #include "base/memory/scoped_ptr.h"
 #include "components/gcm_driver/fake_gcm_driver.h"
 #include "google_apis/gaia/fake_identity_provider.h"
 #include "google_apis/gaia/fake_oauth2_token_service.h"
 #include "google_apis/gaia/google_service_auth_error.h"
 #include "net/http/http_status_code.h"
 #include "net/url_request/test_url_fetcher_factory.h"
 #include "net/url_request/url_request_test_util.h"
 #include "testing/gtest/include/gtest/gtest.h"
 
 namespace gcm {
 
 namespace {
 
 const char kAccountId1[] = "account_1";
 const char kAccountId2[] = "account_2";
 
-std::string AccountKeyToObfuscatedId(const std::string email) {
+std::string AccountKeyToObfuscatedId(const std::string& email) {
   return "obfid-" + email;
 }
 
-std::string GetValidTokenInfoResponse(const std::string account_key) {
+std::string GetValidTokenInfoResponse(const std::string& account_key) {
   return std::string("{ \"id\": \"") + AccountKeyToObfuscatedId(account_key) +
          "\" }";
 }
 
 std::string MakeAccessToken(const std::string& account_key) {
   return "access_token-" + account_key;
 }
 
 GCMClient::AccountTokenInfo MakeAccountToken(const std::string& account_key) {
   GCMClient::AccountTokenInfo token_info;
@@ skipping 469 matching lines @@
   // but we have the update call.
   SignOutAccount(kAccountId1);
   EXPECT_TRUE(driver()->update_accounts_called());
   EXPECT_FALSE(IsTokenReportingRequired());
 }
 
 // TODO(fgorski): Add test for adding account after removal >> make sure it does
 // not mark removal.
 
 }  // namespace gcm