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_interval_set.h b/quic/core/quic_interval_set.h
index e2ca7a5..fe3165b 100644
--- a/quic/core/quic_interval_set.h
+++ b/quic/core/quic_interval_set.h
@@ -298,7 +298,7 @@
   // where the intervals are in the same order as given by traversal from
   // begin() to end(). This representation is intended for human consumption;
   // computer programs should not rely on the output being in exactly this form.
-  QuicString ToString() const;
+  std::string ToString() const;
 
   QuicIntervalSet& operator=(std::initializer_list<value_type> il) {
     assign(il.begin(), il.end());
@@ -809,7 +809,7 @@
 }
 
 template <typename T>
-QuicString QuicIntervalSet<T>::ToString() const {
+std::string QuicIntervalSet<T>::ToString() const {
   std::ostringstream os;
   os << *this;
   return os.str();