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

sync/internal_api/sync_encryption_handler_impl_unittest.cc

Index: sync/internal_api/sync_encryption_handler_impl_unittest.cc
diff --git a/sync/internal_api/sync_encryption_handler_impl_unittest.cc b/sync/internal_api/sync_encryption_handler_impl_unittest.cc
index 1f18a0844bb66711587b7d644fbc825d7575e132..31ae061a1815188a6b49d280d932792cb2782721 100644
--- a/sync/internal_api/sync_encryption_handler_impl_unittest.cc
+++ b/sync/internal_api/sync_encryption_handler_impl_unittest.cc
@@ -64,7 +64,7 @@ class SyncEncryptionHandlerObserverMock
 };
 
 google::protobuf::RepeatedPtrField<google::protobuf::string>
-BuildEncryptionKeyProto(std::string encryption_key) {
+BuildEncryptionKeyProto(const std::string& encryption_key) {
   google::protobuf::RepeatedPtrField<google::protobuf::string> keys;
   keys.Add()->assign(encryption_key);
   return keys;