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/quic_sent_packet_manager.cc b/quic/core/quic_sent_packet_manager.cc
index 6d6710f..bfafe43 100644
--- a/quic/core/quic_sent_packet_manager.cc
+++ b/quic/core/quic_sent_packet_manager.cc
@@ -1010,7 +1010,7 @@
   return retransmission_delay;
 }
 
-QuicString QuicSentPacketManager::GetDebugState() const {
+std::string QuicSentPacketManager::GetDebugState() const {
   return send_algorithm_->GetDebugState();
 }