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

sync/util/cryptographer.h

 // Copyright 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.
 
 #ifndef SYNC_UTIL_CRYPTOGRAPHER_H_
 #define SYNC_UTIL_CRYPTOGRAPHER_H_
 
 #include <map>
 #include <string>
 
@@ skipping 99 matching lines @@
   // Will decrypt the pending keys and install them if possible (pending key
   // will not overwrite default).
   bool AddKey(const KeyParams& params);
 
   // Same as AddKey(..), but builds the new Nigori from a previously persisted
   // bootstrap token. This can be useful when consuming a bootstrap token
   // with a cryptographer that has already been initialized.
   // Updates the default key.
   // Will decrypt the pending keys and install them if possible (pending key
   // will not overwrite default).
-  bool AddKeyFromBootstrapToken(const std::string restored_bootstrap_token);
+  bool AddKeyFromBootstrapToken(const std::string& restored_bootstrap_token);
 
   // Creates a new Nigori instance using |params|. If successful, |params|
   // will be added to the nigori keybag, but will not be the default encryption
   // key (default_nigori_ will remain the same).
   // Prereq: is_initialized() must be true.
   // Will decrypt the pending keys and install them if possible (pending key
   // will become the new default).
   bool AddNonDefaultKey(const KeyParams& params);
 
   // Decrypts |encrypted| and uses its contents to initialize Nigori instances.
@@ skipping 48 matching lines @@
 
   // Returns the name of the Nigori key currently used for encryption.
   std::string GetDefaultNigoriKeyName() const;
 
   // Returns a serialized sync_pb::NigoriKey version of current default
   // encryption key.
   std::string GetDefaultNigoriKeyData() const;
 
   // Generates a new Nigori from |serialized_nigori_key|, and if successful
   // installs the new nigori as the default key.
-  bool ImportNigoriKey(const std::string serialized_nigori_key);
+  bool ImportNigoriKey(const std::string& serialized_nigori_key);
 
  private:
   typedef std::map<std::string, linked_ptr<const Nigori> > NigoriMap;
 
   // Helper method to instantiate Nigori instances for each set of key
   // parameters in |bag|.
   // Does not update the default nigori.
   void InstallKeyBag(const sync_pb::NigoriKeyBag& bag);
 
   // Helper method to add a nigori to the keybag, optionally making it the
@@ skipping 13 matching lines @@
   std::string default_nigori_name_;
 
   scoped_ptr<sync_pb::EncryptedData> pending_keys_;
 
   DISALLOW_ASSIGN(Cryptographer);
 };
 
 }  // namespace syncer
 
 #endif  // SYNC_UTIL_CRYPTOGRAPHER_H_