Ensures that audio tasks cannot run after AudioManager is deleted.

media/audio/BUILD.gn

Index: media/audio/BUILD.gn
diff --git a/media/audio/BUILD.gn b/media/audio/BUILD.gn
index f7d9a5102aef2fb3fb939c5f88f39db181dcd3fc..1a95aefbaaecdaab11ab9a863a7131310e4d5cff 100644
--- a/media/audio/BUILD.gn
+++ b/media/audio/BUILD.gn
@@ -104,6 +104,9 @@ source_set("audio") {
     "audio_system.h",
     "audio_system_impl.cc",
     "audio_system_impl.h",
+    "audio_thread.h",
+    "audio_thread_impl.cc",
+    "audio_thread_impl.h",
     "clockless_audio_sink.cc",
     "clockless_audio_sink.h",
     "fake_audio_input_stream.cc",
@@ -297,6 +300,8 @@ static_library("test_support") {
     "mock_audio_source_callback.h",
     "test_audio_input_controller_factory.cc",
     "test_audio_input_controller_factory.h",
+    "test_audio_thread.cc",
+    "test_audio_thread.h",
   ]
   configs += [ ":platform_config" ]
   deps = [