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_data_writer.cc b/quic/core/quic_data_writer.cc
index c70bbdd..98a173d 100644
--- a/quic/core/quic_data_writer.cc
+++ b/quic/core/quic_data_writer.cc
@@ -349,7 +349,7 @@
   return true;
 }
 
-QuicString QuicDataWriter::DebugString() const {
+std::string QuicDataWriter::DebugString() const {
   return QuicStrCat(" { capacity: ", capacity_, ", length: ", length_, " }");
 }