Use std::string instead of absl::string_view in CryptoBufferMap.

PiperOrigin-RevId: 431166745

quic/test_tools/simple_data_producer.h

diff --git a/quic/test_tools/simple_data_producer.h b/quic/test_tools/simple_data_producer.h
index 8111c1c..dc0c076 100644
--- a/quic/test_tools/simple_data_producer.h
+++ b/quic/test_tools/simple_data_producer.h
@@ -55,7 +55,7 @@
 
   using CryptoBufferMap =
       absl::flat_hash_map<std::pair<EncryptionLevel, QuicStreamOffset>,
-                          absl::string_view>;
+                          std::string>;
 
   SimpleBufferAllocator allocator_;