Remove public visibility from some rtc_base libraries.

Bug: webrtc:9838
Change-Id: I34943a8f9422284a2e4c7ec287e7a294fc120db1
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/129445
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#27275}

rtc_base/BUILD.gn

@@ -49,7 +49,6 @@ if (!rtc_build_ssl) {
 }
 
 rtc_source_set("protobuf_utils") {
-  visibility = [ "*" ]
   sources = [
     "protobuf_utils.h",
   ]
@@ -1025,7 +1024,6 @@ rtc_static_library("rtc_base") {
 }
 
 rtc_source_set("gtest_prod") {
-  visibility = [ "*" ]
   sources = [
     "gtest_prod_util.h",
   ]
@@ -1275,7 +1273,6 @@ if (rtc_include_tests) {
   }
 
   rtc_source_set("rtc_task_queue_unittests") {
-    visibility = [ "*" ]
     testonly = true
 
     sources = [