GN: Tighten up test target visibility + refactorings

webrtc/modules/audio_mixer/BUILD.gn

Index: webrtc/modules/audio_mixer/BUILD.gn
diff --git a/webrtc/modules/audio_mixer/BUILD.gn b/webrtc/modules/audio_mixer/BUILD.gn
index b38769c882bbcc006af4c7ee732c284cb99e9ea1..56696c49682ef75b5833ae790a82e94f5eee4a71 100644
--- a/webrtc/modules/audio_mixer/BUILD.gn
+++ b/webrtc/modules/audio_mixer/BUILD.gn
@@ -66,6 +66,9 @@ rtc_static_library("audio_frame_manipulator") {
 if (rtc_include_tests) {
   rtc_source_set("audio_mixer_unittests") {
     testonly = true
+    if (!is_android && !is_ios) {  # Generated targets makes this check break.
+      visibility = [ "//webrtc/modules:modules_unittests" ]
+    }
     sources = [
       "audio_frame_manipulator_unittest.cc",
       "audio_mixer_impl_unittest.cc",