GN: Tighten up test target visibility + refactorings

webrtc/modules/audio_conference_mixer/BUILD.gn

 # Copyright (c) 2014 The WebRTC project authors. All Rights Reserved.
 #
 # Use of this source code is governed by a BSD-style license
 # that can be found in the LICENSE file in the root of the source
 # tree. An additional intellectual property rights grant can be found
 # in the file PATENTS.  All contributing project authors may
 # be found in the AUTHORS file in the root of the source tree.
 
 import("../../webrtc.gni")
 
@@ skipping 32 matching lines @@
     "../../audio/utility:audio_frame_operations",
     "../../base:rtc_base_approved",
     "../../system_wrappers",
     "../audio_processing",
   ]
 }
 
 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",
     ]
     deps = [
       ":audio_conference_mixer",
       "../../test:test_support",
       "//testing/gmock",
     ]
     if (is_win) {
       cflags = [
         # TODO(kjellander): bugs.webrtc.org/261: Fix this warning.
         "/wd4373",  # virtual function override.
       ]
     }
     if (!build_with_chromium && is_clang) {
       # Suppress warnings from the Chromium Clang plugin (bugs.webrtc.org/163).
       suppressed_configs += [ "//build/config/clang:find_bad_constructs" ]
     }
   }
 }