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

net/filter/sdch_filter_unittest.cc

Index: net/filter/sdch_filter_unittest.cc
diff --git a/net/filter/sdch_filter_unittest.cc b/net/filter/sdch_filter_unittest.cc
index a83e63ff4c9b0971f97428d432cb2e3a1fcc1d77..35276cb764a5b22e8106de00b4ea3fde0be675b3 100644
--- a/net/filter/sdch_filter_unittest.cc
+++ b/net/filter/sdch_filter_unittest.cc
@@ -85,7 +85,7 @@ class SdchFilterTest : public testing::Test {
         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);