Add 100 ms network delay to the SupportsFlexFEC* tests.

Some of the tests are currently flaky because FEC is disabled if the
RTT is <200 ms, and the simulated network is configured to use 100 ms
for the send transport, but nothing is configured for the receive
transport. This CL configures the receive transport to 100 ms delay.

Bug: webrtc:10920
Change-Id: I79995693ba73683406fa9ced92a7918e6c05473f
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/154571
Commit-Queue: Björn Terelius <terelius@webrtc.org>
Reviewed-by: Sebastian Jansson <srte@webrtc.org>
Reviewed-by: Rasmus Brandt <brandtr@webrtc.org>
Reviewed-by: Erik Språng <sprang@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#29394}

video/video_send_stream_tests.cc

@@ -769,6 +769,21 @@ class FlexfecObserver : public test::EndToEndTest {
             std::make_unique<SimulatedNetwork>(config)));
   }
 
+  std::unique_ptr<test::PacketTransport> CreateReceiveTransport(
+      TaskQueueBase* task_queue) override {
+    // We need the RTT to be >200 ms to send FEC and the network delay for the
+    // send transport is 100 ms, so add 100 ms (but no loss) on the return link.
+    BuiltInNetworkBehaviorConfig config;
+    config.loss_percent = 0;
+    config.queue_delay_ms = 100;
+    return std::make_unique<test::PacketTransport>(
+        task_queue, nullptr, this, test::PacketTransport::kReceiver,
+        VideoSendStreamTest::payload_type_map_,
+        std::make_unique<FakeNetworkPipe>(
+            Clock::GetRealTimeClock(),
+            std::make_unique<SimulatedNetwork>(config)));
+  }
+
   void ModifyVideoConfigs(
       VideoSendStream::Config* send_config,
       std::vector<VideoReceiveStream::Config>* receive_configs,