Move nat-related code to target rtc_base_tests_utils.

webrtc/base/BUILD.gn

Index: webrtc/base/BUILD.gn
diff --git a/webrtc/base/BUILD.gn b/webrtc/base/BUILD.gn
index b46e4c2a79acc902803f733ae2da0db26e413297..60ac7d4e4ff8e11fda776b7c71320091f91a5806 100644
--- a/webrtc/base/BUILD.gn
+++ b/webrtc/base/BUILD.gn
@@ -533,12 +533,6 @@ rtc_static_library("rtc_base") {
       "logsinks.cc",
       "logsinks.h",
       "mathutils.h",
-      "natserver.cc",
-      "natserver.h",
-      "natsocketfactory.cc",
-      "natsocketfactory.h",
-      "nattypes.cc",
-      "nattypes.h",
       "optionsfile.cc",
       "optionsfile.h",
       "proxyserver.cc",
@@ -726,6 +720,12 @@ if (rtc_include_tests) {
       "firewallsocketserver.cc",
       "firewallsocketserver.h",
       "gunit.h",
+      "natserver.cc",
+      "natserver.h",
+      "natsocketfactory.cc",
+      "natsocketfactory.h",
+      "nattypes.cc",
+      "nattypes.h",
       "sigslottester.h",
       "sigslottester.h.pump",
       "testbase64.h",