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

media/cast/common/transport_encryption_handler.cc

Index: media/cast/common/transport_encryption_handler.cc
diff --git a/media/cast/common/transport_encryption_handler.cc b/media/cast/common/transport_encryption_handler.cc
index 54a43e8b52647fbd04d36ed1a2097e99e84b5382..360e40c63a8c48833bc35718842e0cc379e606a9 100644
--- a/media/cast/common/transport_encryption_handler.cc
+++ b/media/cast/common/transport_encryption_handler.cc
@@ -41,8 +41,8 @@ TransportEncryptionHandler::TransportEncryptionHandler()
 
 TransportEncryptionHandler::~TransportEncryptionHandler() {}
 
-bool TransportEncryptionHandler::Initialize(std::string aes_key,
-                                            std::string aes_iv_mask) {
+bool TransportEncryptionHandler::Initialize(const std::string& aes_key,
+                                            const std::string& aes_iv_mask) {
   is_activated_ = false;
   if (aes_iv_mask.size() == kAesKeySize && aes_key.size() == kAesKeySize) {
     iv_mask_ = aes_iv_mask;