Remove som GYP annotations, remove unused manifest.

chromecast/media/cma/backend/alsa/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.
 
 import("//build/buildflag_header.gni")
 import("//build/config/chromecast_build.gni")
 import("//chromecast/chromecast.gni")
 import("//media/media_options.gni")
 import("//testing/test.gni")
 
 declare_args() {
   # Set true to use raw timestamps on non-desktop cast build.
   # ALSA version equal or later than 1.0.29 support this function.
   use_alsa_monotonic_raw_tstamps = !is_cast_desktop_build
 }
 
 # Alsa must be used for these targets to build.
 assert(use_alsa)
 
-# GYP target: chromecast/media/media.gyp:libcast_media_1.0_audio
 shared_library("libcast_media_1.0_audio") {
   sources = [
     "cast_media_shlib.cc",
     "media_codec_support_cast_audio.cc",
   ]
 
   deps = [
     ":alsa_cma_backend",
     "//base",
     "//chromecast/base",
     "//chromecast/public",
     "//media",
   ]
 }
 
-# GYP target: chromecast/media/media.gyp:alsa_cma_backend
 source_set("alsa_cma_backend") {
   sources = [
     "alsa_wrapper.cc",
     "alsa_wrapper.h",
     "audio_decoder_alsa.cc",
     "audio_decoder_alsa.h",
     "media_pipeline_backend_alsa.cc",
     "media_pipeline_backend_alsa.h",
     "stream_mixer_alsa.cc",
     "stream_mixer_alsa.h",
@@ skipping 52 matching lines @@
   sources = [
     "slew_volume.cc",
     "slew_volume.h",
   ]
   deps = [
     "//base",
     "//media",
   ]
 }
 
-# GYP target: chromecast/media/media.gyp:chromecast_alsa_features
 buildflag_header("alsa_features") {
   header = "alsa_features.h"
 
   flags = [ "ALSA_MONOTONIC_RAW_TSTAMPS=$use_alsa_monotonic_raw_tstamps" ]
 }
 
-# GYP target: chromecast/media/media.gyp:cast_alsa_cma_backend_unittests
 test("cast_alsa_cma_backend_unittests") {
   sources = [
     "stream_mixer_alsa_unittest.cc",
   ]
 
   deps = [
     ":test_support",
     "//base",
     "//base/test:run_all_unittests",
     "//chromecast/media/base",
     "//chromecast/media/base:libcast_media_1.0_default_core",
     "//media",
     "//media:shared_memory_support",
     "//testing/gmock",
     "//testing/gtest",
   ]
 }
 
-# GYP target: n/a
 source_set("test_support") {
   testonly = true
 
   sources = [
     "mock_alsa_wrapper.cc",
     "mock_alsa_wrapper.h",
   ]
 
   public_deps = [
     ":alsa_cma_backend",
   ]
 
   deps = [
     "//base",
     "//media",
     "//testing/gmock",
   ]
 }