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.h b/quic/core/quic_sent_packet_manager.h
index aacdc34..873e88b 100644
--- a/quic/core/quic_sent_packet_manager.h
+++ b/quic/core/quic_sent_packet_manager.h
@@ -237,7 +237,7 @@
   }
 
   // Returns debugging information about the state of the congestion controller.
-  QuicString GetDebugState() const;
+  std::string GetDebugState() const;
 
   // Returns the number of bytes that are considered in-flight, i.e. not lost or
   // acknowledged.