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

chrome/browser/safe_browsing/client_side_detection_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 <map>
 #include <queue>
 #include <string>
 
 #include "base/bind.h"
 #include "base/callback.h"
@@ skipping 20 matching lines @@
 using ::testing::Mock;
 using ::testing::StrictMock;
 using ::testing::_;
 using content::BrowserThread;
 
 namespace safe_browsing {
 namespace {
 
 class MockModelLoader : public ModelLoader {
  public:
-  explicit MockModelLoader(const std::string model_name)
+  explicit MockModelLoader(const std::string& model_name)
       : ModelLoader(base::Closure(), model_name) {}
   ~MockModelLoader() override {}
 
   MOCK_METHOD1(ScheduleFetch, void(int64));
   MOCK_METHOD0(CancelFetcher, void());
 
  private:
   DISALLOW_COPY_AND_ASSIGN(MockModelLoader);
 };
 
@@ skipping 425 matching lines @@
 
   // Check that disabling again doesn't request the model.
   csd_service_->SetEnabledAndRefreshState(false);
   // No calls expected.
   msg_loop_.RunUntilIdle();
   Mock::VerifyAndClearExpectations(service);
   Mock::VerifyAndClearExpectations(loader_1);
   Mock::VerifyAndClearExpectations(loader_2);
 }
 }  // namespace safe_browsing