GN: Tighten up test target visibility + refactorings

webrtc/modules/audio_conference_mixer/BUILD.gn

Index: webrtc/modules/audio_conference_mixer/BUILD.gn
diff --git a/webrtc/modules/audio_conference_mixer/BUILD.gn b/webrtc/modules/audio_conference_mixer/BUILD.gn
index 428bcac03cd950261038f2e0bdc9feb7e1999b13..fc9904c23bc78596f88844f705c0ae626a955a86 100644
--- a/webrtc/modules/audio_conference_mixer/BUILD.gn
+++ b/webrtc/modules/audio_conference_mixer/BUILD.gn
@@ -50,6 +50,13 @@ rtc_static_library("audio_conference_mixer") {
 if (rtc_include_tests) {
   rtc_source_set("audio_conference_mixer_unittests") {
     testonly = true
+
+    # Skip restricting visibility on mobile platforms since the tests on those
+    # gets additional generated targets which would require many lines here to
+    # cover (which would be confusing to read and hard to maintain).
+    if (!is_android && !is_ios) {
+      visibility = [ "//webrtc/modules:modules_unittests" ]
+    }
     sources = [
       "test/audio_conference_mixer_unittest.cc",
     ]