GN: Tighten up test target visibility + refactorings

webrtc/modules/audio_device/BUILD.gn

Index: webrtc/modules/audio_device/BUILD.gn
diff --git a/webrtc/modules/audio_device/BUILD.gn b/webrtc/modules/audio_device/BUILD.gn
index da05f5f11f4dfb769579da76b1e142dacb3133e0..1e691fa9bc48e25bf64f20f7e27053fa28800d09 100644
--- a/webrtc/modules/audio_device/BUILD.gn
+++ b/webrtc/modules/audio_device/BUILD.gn
@@ -255,6 +255,13 @@ config("mock_audio_device_config") {
 if (rtc_include_tests) {
   rtc_source_set("audio_device_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 = [
       "fine_audio_buffer_unittest.cc",
     ]