[Chromecast] Add new volume control API to CastMediaShlib

chromecast/public/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.
 
 config("public_config") {
   include_dirs = [ "//chromecast/public" ]
 }
 
 source_set("public") {
   sources = [
     "avsettings.h",
     "cast_egl_platform.h",
     "cast_egl_platform_shlib.h",
     "cast_media_shlib.h",
     "cast_sys_info.h",
     "cast_sys_info_shlib.h",
     "chromecast_export.h",
     "graphics_properties_shlib.h",
     "graphics_types.h",
     "media_codec_support_shlib.h",
     "osd_plane.h",
     "osd_plane_shlib.h",
     "osd_surface.h",
     "reboot_shlib.h",
     "task_runner.h",
     "video_plane.h",
+    "volume_control.h",
   ]
 
   public_configs = [ ":public_config" ]
 }