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

net/filter/sdch_filter_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 <limits.h>
 
 #include <algorithm>
 #include <string>
 #include <vector>
 
@@ skipping 67 matching lines @@
 
   MockFilterContext* filter_context() { return filter_context_.get(); }
 
   // Sets both the GURL and the SDCH response for a filter context.
   void SetupFilterContextWithGURL(GURL url) {
     filter_context_->SetURL(url);
     filter_context_->SetSdchResponse(
         sdch_manager_->GetDictionarySet(url).Pass());
   }
 
-  std::string NewSdchCompressedData(const std::string dictionary) {
+  std::string NewSdchCompressedData(const std::string& dictionary) {
     std::string client_hash;
     std::string server_hash;
     SdchManager::GenerateHash(dictionary, &client_hash, &server_hash);
 
     // Build compressed data that refers to our dictionary.
     std::string compressed(server_hash);
     compressed.append("\0", 1);
     compressed.append(vcdiff_compressed_data_);
     return compressed;
   }
@@ skipping 1193 matching lines @@
 
   filter.reset(nullptr);
 
   // Confirm that we got a "DictionaryUsed" signal from the SdchManager
   // for our dictionary.
   EXPECT_EQ(1, observer.dictionary_used_calls());
   EXPECT_EQ(server_hash, observer.last_server_hash());
 }
 
 }  // namespace net