Allow injection of custom network models in place of FakeNetworkPipe.

Adds a constructor for DirectTransport that takes a pointer to an instance 
of a class derived from FakeNetworkPipe. Said class can override Process() 
and SendPacket(...) members thereby emulating any desired network behavior.

Bug: b/67487983
Change-Id: I829fd3506124db61587af19192a14fdf62b06ca5
Reviewed-on: https://webrtc-review.googlesource.com/14620
Commit-Queue: Christoffer Rodbro <crodbro@webrtc.org>
Reviewed-by: Stefan Holmer <stefan@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#20443}

test/direct_transport.h

diff --git a/test/direct_transport.h b/test/direct_transport.h
index 68a1968..85cd892 100644
--- a/test/direct_transport.h
+++ b/test/direct_transport.h
@@ -46,6 +46,9 @@
                   Call* send_call,
                   std::unique_ptr<Demuxer> demuxer);
 
+  DirectTransport(SingleThreadedTaskQueueForTesting* task_queue,
+                  std::unique_ptr<FakeNetworkPipe> pipe, Call* send_call);
+
   ~DirectTransport() override;
 
   void SetConfig(const FakeNetworkPipe::Config& config);
@@ -64,6 +67,7 @@
 
  private:
   void SendPackets();
+  void Start();
 
   Call* const send_call_;
   Clock* const clock_;
@@ -72,7 +76,7 @@
   SingleThreadedTaskQueueForTesting::TaskId next_scheduled_task_
       RTC_GUARDED_BY(&sequence_checker_);
 
-  FakeNetworkPipe fake_network_;
+  std::unique_ptr<FakeNetworkPipe> fake_network_;
 
   rtc::SequencedTaskChecker sequence_checker_;
 };