rtc_test visibility must only be public.

"rtc_test" expands to "test" which is another template. The "test"
template overrides the visibility to public when targeting Android.

WebRTC wants to avoid this kind of situations and since a test should
be considered like an executable it is ok to force to public.

Bug: None
Change-Id: Iaf382721c60bb7345d4599a0279f94a4653247ec
No-Try: True
Reviewed-on: https://webrtc-review.googlesource.com/24560
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#20813}

webrtc.gni

@@ -266,7 +266,10 @@ template("rtc_test") {
                              "suppressed_configs",
                              "visibility",
                            ])
-    forward_variables_from(invoker, [ "visibility" ])
+    assert(
+        !defined(invoker.visibility),
+        "rtc_test always has visibility `*`; please don't specify it manually")
+    visibility = [ "*" ]
     configs += invoker.configs
     configs -= rtc_remove_configs
     configs -= invoker.suppressed_configs