Add ramp-up tests for transport sequence number with and w/o audio.

webrtc/test/direct_transport.cc

Index: webrtc/test/direct_transport.cc
diff --git a/webrtc/test/direct_transport.cc b/webrtc/test/direct_transport.cc
index 6f7f998cf23fd666d3e428c87b29e251a977553c..591e154b1406b3dc4fe088bb4aa9d006dd54f13a 100644
--- a/webrtc/test/direct_transport.cc
+++ b/webrtc/test/direct_transport.cc
@@ -70,6 +70,10 @@ bool DirectTransport::SendRtcp(const uint8_t* data, size_t length) {
   return true;
 }
 
+int DirectTransport::GetAverageDelayMs() {
+  return fake_network_.AverageDelay();
+}
+
 bool DirectTransport::NetworkProcess(void* transport) {
   return static_cast<DirectTransport*>(transport)->SendPackets();
 }