Use SingleThreadedTaskQueue in DirectTransport

DirectTransport has so far used its own thread, which led to a different threading-model for in the unit-tests than is used in actual WebRTC. Because of that, some critical-sections that weren't truly necessary in WebRTC could not be replaced with thread-checks, because those checks failed in unit-tests.

This CL introduces SingleThreadedTaskQueue - a TaskQueue which guarantees to run all of its tasks on the same thread (rtc::TaskQueue doesn't guarantee that on Mac) - and uses that for DirectTransport. CLs based on top of this will uncomment thread-checks which had to be commented out before, and remove unnecessary critical-sections.

Future work would probably replace the thread-checkers by more sophisticated serialized-access checks, allowing us to move from the SingleThreadedTaskQueue to a normal TaskQueue.

Related implementation notes:
* This CL has made DirectTransport::StopSending() superfluous, and so it was deleted.

BUG=webrtc:8113, webrtc:7405, webrtc:8056, webrtc:8116

Review-Url: https://codereview.webrtc.org/2998923002
Cr-Commit-Position: refs/heads/master@{#19445}
diff --git a/webrtc/call/rampup_tests.cc b/webrtc/call/rampup_tests.cc
index a50abee..cfb5117 100644
--- a/webrtc/call/rampup_tests.cc
+++ b/webrtc/call/rampup_tests.cc
@@ -90,9 +90,11 @@
   send_stream_ = send_stream;
 }
 
-test::PacketTransport* RampUpTester::CreateSendTransport(Call* sender_call) {
+test::PacketTransport* RampUpTester::CreateSendTransport(
+    test::SingleThreadedTaskQueueForTesting* task_queue,
+    Call* sender_call) {
   send_transport_ = new test::PacketTransport(
-      sender_call, this, test::PacketTransport::kSender,
+      task_queue, sender_call, this, test::PacketTransport::kSender,
       test::CallTest::payload_type_map_, forward_transport_config_);
   return send_transport_;
 }