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_bandwidth.h b/quic/core/quic_bandwidth.h
index be3c158..1160d92 100644
--- a/quic/core/quic_bandwidth.h
+++ b/quic/core/quic_bandwidth.h
@@ -86,7 +86,7 @@
                                              bits_per_second_);
   }
 
-  QuicString ToDebugValue() const;
+  std::string ToDebugValue() const;
 
  private:
   explicit constexpr QuicBandwidth(int64_t bits_per_second)