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_connection_id.cc b/quic/core/quic_connection_id.cc
index 25c5b1f..07f3a67 100644
--- a/quic/core/quic_connection_id.cc
+++ b/quic/core/quic_connection_id.cc
@@ -64,9 +64,9 @@
                                  data_bytes[0] ^ data_bytes[1] ^ data_bytes[2]);
 }
 
-QuicString QuicConnectionId::ToString() const {
+std::string QuicConnectionId::ToString() const {
   if (IsEmpty()) {
-    return QuicString("0");
+    return std::string("0");
   }
   return QuicTextUtils::HexEncode(data_, length_);
 }