Replace QuicString with std::string, pass 1

This replaces QuicString with std::string in all of the "QUIC proper".  I will
delete QuicString once all code using it is gone.

gfe-relnote: n/a (no functional change)
PiperOrigin-RevId: 237872023
Change-Id: I82de62c9855516b15039734d05155917e68ff4ee
diff --git a/quic/core/crypto/aead_base_decrypter.cc b/quic/core/crypto/aead_base_decrypter.cc
index 53bd0f0..474c443 100644
--- a/quic/core/crypto/aead_base_decrypter.cc
+++ b/quic/core/crypto/aead_base_decrypter.cc
@@ -125,7 +125,7 @@
     return true;
   }
 
-  QuicString key, nonce_prefix;
+  std::string key, nonce_prefix;
   size_t prefix_size = nonce_size_ - sizeof(QuicPacketNumber);
   DiversifyPreliminaryKey(
       QuicStringPiece(reinterpret_cast<const char*>(key_), key_size_),