Add copy assignment operator to supress -Wdeprecated-copy warning.

This is copied over from
https://quiche-review.googlesource.com/c/quiche/+/10760.

PiperOrigin-RevId: 381523102
diff --git a/quic/core/congestion_control/bandwidth_sampler.h b/quic/core/congestion_control/bandwidth_sampler.h
index 6738f49..2d5924b 100644
--- a/quic/core/congestion_control/bandwidth_sampler.h
+++ b/quic/core/congestion_control/bandwidth_sampler.h
@@ -47,6 +47,7 @@
         bytes_in_flight(bytes_in_flight) {}
 
   SendTimeState(const SendTimeState& other) = default;
+  SendTimeState& operator=(const SendTimeState& other) = default;
 
   friend QUIC_EXPORT_PRIVATE std::ostream& operator<<(std::ostream& os,
                                                       const SendTimeState& s);