dcsctp: Specify an initial RTT

The RTT will quickly be updated whenever a DATA chunk is acked, but
the initial value was set to zero. In unit tests, which often are
short and rarely increase the simulated time between a DATA is sent,
and acked, the smoothed RTT would usually stay at 0, which causes
e.g. the rate limiting of FORWARD not to work.

Bug: None
Change-Id: Ieb515fe875ce88d001777b00d6efd9762565a09d
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/273900
Reviewed-by: Florent Castelli <orphis@webrtc.org>
Commit-Queue: Victor Boivie <boivie@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#38013}

net/dcsctp/tx/retransmission_timeout.cc

@@ -21,6 +21,7 @@ RetransmissionTimeout::RetransmissionTimeout(const DcSctpOptions& options)
       max_rto_(*options.rto_max),
       max_rtt_(*options.rtt_max),
       min_rtt_variance_(*options.min_rtt_variance),
+      scaled_srtt_(*options.rto_initial << kRttShift),
       rto_(*options.rto_initial) {}
 
 void RetransmissionTimeout::ObserveRTT(DurationMs measured_rtt) {

net/dcsctp/tx/retransmission_timeout.h

@@ -48,7 +48,7 @@ class RetransmissionTimeout {
   // If this is the first measurement
   bool first_measurement_ = true;
   // Smoothed Round-Trip Time, shifted by kRttShift
-  int32_t scaled_srtt_ = 0;
+  int32_t scaled_srtt_;
   // Round-Trip Time Variation, shifted by kRttVarShift
   int32_t scaled_rtt_var_ = 0;
   // Retransmission Timeout

net/dcsctp/tx/retransmission_timeout_test.cc

@@ -37,6 +37,11 @@ TEST(RetransmissionTimeoutTest, HasValidInitialRto) {
   EXPECT_EQ(rto_.rto(), kInitialRto);
 }
 
+TEST(RetransmissionTimeoutTest, HasValidInitialSrtt) {
+  RetransmissionTimeout rto_(MakeOptions());
+  EXPECT_EQ(rto_.srtt(), kInitialRto);
+}
+
 TEST(RetransmissionTimeoutTest, NegativeValuesDoNotAffectRTO) {
   RetransmissionTimeout rto_(MakeOptions());
   // Initial negative value