GN: Tighten up test target visibility + refactorings

webrtc/p2p/BUILD.gn

Index: webrtc/p2p/BUILD.gn
diff --git a/webrtc/p2p/BUILD.gn b/webrtc/p2p/BUILD.gn
index a31b7f1e4929f169b8930b587c8ca473d1422d58..d7e5f9dd9ee94dba7e417aed49e5ac3d77dcaa4b 100644
--- a/webrtc/p2p/BUILD.gn
+++ b/webrtc/p2p/BUILD.gn
@@ -169,6 +169,9 @@ if (rtc_include_tests) {
 
   rtc_source_set("rtc_p2p_unittests") {
     testonly = true
+    if (!is_android && !is_ios) {  # Generated targets makes this check break.
+      visibility = [ "//webrtc:rtc_unittests" ]
+    }
     sources = [
       "base/asyncstuntcpsocket_unittest.cc",
       "base/dtlstransportchannel_unittest.cc",
@@ -236,6 +239,9 @@ rtc_static_library("libstunprober") {
 if (rtc_include_tests) {
   rtc_source_set("libstunprober_unittests") {
     testonly = true
+    if (!is_android && !is_ios) {  # Generated targets makes this check break.
+      visibility = [ "//webrtc:rtc_unittests" ]
+    }
     sources = [
       "stunprober/stunprober_unittest.cc",
     ]