Decompose //extensions/browser/BUILD.gn

extensions/browser/api/audio/BUILD.gn

+# Copyright 2016 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.
+
+source_set("audio") {
+  sources = [
+    "audio_api.cc",
+    "audio_api.h",
+    "audio_service.h",
+  ]
+
+  if (is_chromeos) {
+    sources += [ "audio_service_chromeos.cc" ]
+  } else {
+    sources += [ "audio_service.cc" ]
+  }
+
+  deps = [
+    "//extensions/common/api",
+  ]
+}