Fix implicit conversion from absl::string_view to std::string.

This does not work in Chromium.

PiperOrigin-RevId: 431976045

quic/test_tools/simple_data_producer.cc

diff --git a/quic/test_tools/simple_data_producer.cc b/quic/test_tools/simple_data_producer.cc
index 413a840..aaab1d1 100644
--- a/quic/test_tools/simple_data_producer.cc
+++ b/quic/test_tools/simple_data_producer.cc
@@ -37,7 +37,7 @@
                                         QuicStreamOffset offset,
                                         absl::string_view data) {
   auto key = std::make_pair(level, offset);
-  crypto_buffer_map_[key] = data;
+  crypto_buffer_map_[key] = std::string(data);
 }
 
 WriteStreamDataResult SimpleDataProducer::WriteStreamData(