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

sync/util/cryptographer.h

Index: sync/util/cryptographer.h
diff --git a/sync/util/cryptographer.h b/sync/util/cryptographer.h
index 530995cb05d7067be10557e1ca50ab6a312b3535..07cd705ee01fc18af137afc4eca129d5770bd070 100644
--- a/sync/util/cryptographer.h
+++ b/sync/util/cryptographer.h
@@ -117,7 +117,7 @@ class SYNC_EXPORT Cryptographer {
   // 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
@@ -186,7 +186,7 @@ class SYNC_EXPORT Cryptographer {
 
   // 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;