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/congestion_control/bbr_sender.cc b/quic/core/congestion_control/bbr_sender.cc
index 6a7fe2b..c119135 100644
--- a/quic/core/congestion_control/bbr_sender.cc
+++ b/quic/core/congestion_control/bbr_sender.cc
@@ -851,7 +851,7 @@
   recovery_window_ = std::max(min_congestion_window_, recovery_window_);
 }
 
-QuicString BbrSender::GetDebugState() const {
+std::string BbrSender::GetDebugState() const {
   std::ostringstream stream;
   stream << ExportDebugState();
   return stream.str();
@@ -875,7 +875,7 @@
   return DebugState(*this);
 }
 
-static QuicString ModeToString(BbrSender::Mode mode) {
+static std::string ModeToString(BbrSender::Mode mode) {
   switch (mode) {
     case BbrSender::STARTUP:
       return "STARTUP";