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

media/cast/test/end2end_unittest.cc

Index: media/cast/test/end2end_unittest.cc
diff --git a/media/cast/test/end2end_unittest.cc b/media/cast/test/end2end_unittest.cc
index 1287ef1032f047359df6e3075bc55f954af8093e..e8137c09e6f1347df4da101c5cd5835c8182f116 100644
--- a/media/cast/test/end2end_unittest.cc
+++ b/media/cast/test/end2end_unittest.cc
@@ -82,7 +82,7 @@ static const int kTargetPlayoutDelayMs = 100;
 // receiver.
 static const int kMaxAllowedPlayoutErrorMs = 30;
 
-std::string ConvertFromBase16String(const std::string base_16) {
+std::string ConvertFromBase16String(const std::string& base_16) {
   std::string compressed;
   DCHECK_EQ(base_16.size() % 2, 0u) << "Must be a multiple of 2";
   compressed.reserve(base_16.size() / 2);