Sync more GN unit tests with GYP.

components/audio_modem/BUILD.gn

 # Copyright 2015 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
-static_library("audio_modem") {
+source_set("audio_modem") {
+  public = [
+    "public/audio_modem_types.h",
+    "public/modem.h",
+    "public/whispernet_client.h",
+  ]
   sources = [
     "audio_modem_switches.cc",
     "audio_modem_switches.h",
     "audio_player.h",
     "audio_player_impl.cc",
     "audio_player_impl.h",
     "audio_recorder.h",
     "audio_recorder_impl.cc",
     "audio_recorder_impl.h",
     "constants.cc",
     "modem_impl.cc",
     "modem_impl.h",
-    "public/audio_modem_types.h",
-    "public/modem.h",
-    "public/whispernet_client.h",
   ]
 
   deps = [
     "//base",
     "//content",
     "//media",
     "//media:shared_memory_support",
     "//third_party/webrtc/common_audio",
   ]
 }
 
 source_set("test_support") {
+  testonly = true
   sources = [
     "test/random_samples.cc",
     "test/random_samples.h",
     "test/stub_modem.cc",
     "test/stub_modem.h",
     "test/stub_whispernet_client.cc",
     "test/stub_whispernet_client.h",
   ]
 
+  public_deps = [
+    ":audio_modem",
+  ]
   deps = [
     "//base",
     "//media",
   ]
 }
+
+source_set("unit_tests") {
+  testonly = true
+  sources = [
+    "audio_player_unittest.cc",
+    "audio_recorder_unittest.cc",
+    "modem_unittest.cc",
+  ]
+
+  configs += [ "//build/config/compiler:no_size_t_to_int_warning" ]
+
+  deps = [
+    ":test_support",
+    "//testing/gtest",
+  ]
+}