Move FirewallSocketServer to test code.

BUG=webrtc:6424

Review-Url: https://codereview.webrtc.org/2515443003
Cr-Commit-Position: refs/heads/master@{#15154}
diff --git a/webrtc/base/BUILD.gn b/webrtc/base/BUILD.gn
index d060070..1d25805 100644
--- a/webrtc/base/BUILD.gn
+++ b/webrtc/base/BUILD.gn
@@ -404,8 +404,6 @@
     "filerotatingstream.h",
     "fileutils.cc",
     "fileutils.h",
-    "firewallsocketserver.cc",
-    "firewallsocketserver.h",
     "flags.cc",
     "flags.h",
     "gunit_prod.h",
@@ -754,6 +752,8 @@
       "fakenetwork.h",
       "fakesslidentity.h",
       "faketaskrunner.h",
+      "firewallsocketserver.cc",
+      "firewallsocketserver.h",
       "gunit.h",
       "testbase64.h",
       "testechoserver.h",