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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index 0000000000000000000000000000000000000000..4b346b751a81b632a4ea56d2c9db853dbb6f52c5
--- /dev/null
+++ b/ash/public/interfaces/volume.mojom
@@ -0,0 +1,15 @@
+// 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.

[James Cook, 2016/10/18 23:53:40]

super nit: I might add a quick note about why this has to be delegated to chrome
(chrome controls media, also volume changes need to be propagated to
extensions/apps, like videoconferencing apps).

[msw, 2016/10/19 15:24:10]

I added a note; let me know if it should be changed.

+interface VolumeController {
+  // Mute the audio volume.
+  VolumeMute();
+  // Decrease the audio volume.
+  VolumeDown();
+  // Increase the audio volume.
+  VolumeUp();
+};