Add GUARDED_BY in DirectTransport

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 81f3b697102bd3201bd7a6316ff4277f627cde85..e955c08d35b8cc739dc3e42ff9806da2b10bbc29 100644
--- a/webrtc/test/direct_transport.cc
+++ b/webrtc/test/direct_transport.cc
@@ -94,6 +94,7 @@ void DirectTransport::SetConfig(const FakeNetworkPipe::Config& config) {
 }
 
 void DirectTransport::StopSending() {
+  RTC_DCHECK_CALLED_SEQUENTIALLY(&sequence_checker_);
   task_queue_->CancelTask(next_scheduled_task_);
 }