Use mojo volume interfaces for mash and classic ash.

ash/public/interfaces/volume.mojom

Index: ash/public/interfaces/volume.mojom
diff --git a/ash/public/interfaces/volume.mojom b/ash/public/interfaces/volume.mojom
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+// 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.
+
+module ash.mojom;
+
+// Implemented by Chrome and used by ash to request audio volume changes.
+// Implemented by Chrome because it's currently the only client of audio dbus
+// interfaces (via CrasAudioHandler); this may be simplified if that changes.
+interface VolumeController {
+  // Mute the audio volume.
+  VolumeMute();
+  // Decrease the audio volume.
+  VolumeDown();
+  // Increase the audio volume.
+  VolumeUp();
+};