Introduce network layer.

This CL contains network emulation layer and is a first part of landing
CL https://webrtc-review.googlesource.com/c/src/+/116663

Bug: webrtc:10138
Change-Id: If664b21e9df847aef8144d622d08fc7e9f6608da
Reviewed-on: https://webrtc-review.googlesource.com/c/120406
Commit-Queue: Artem Titov <titovartem@webrtc.org>
Reviewed-by: Sebastian Jansson <srte@webrtc.org>
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#26470}
diff --git a/BUILD.gn b/BUILD.gn
index d9c9d39..8243281 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -487,6 +487,7 @@
       "rtc_base:sigslot_unittest",
       "rtc_base:weak_ptr_unittests",
       "rtc_base/experiments:experiments_unittests",
+      "test/scenario/network:network_emulation_unittests",
     ]
 
     if (rtc_enable_protobuf) {