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_time.h b/quic/core/quic_time.h
index 6bd3f68..9624b6f 100644
--- a/quic/core/quic_time.h
+++ b/quic/core/quic_time.h
@@ -75,7 +75,7 @@
       return time_offset_ == kQuicInfiniteTimeUs;
     }
 
-    QuicString ToDebugValue() const;
+    std::string ToDebugValue() const;
 
    private:
     friend inline bool operator==(QuicTime::Delta lhs, QuicTime::Delta rhs);