mash: Have chrome set itself as a controller interface for changing volume

ash/public/interfaces/accelerator_controller.mojom

 // 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();
+import "ash/public/interfaces/volume.mojom";
+
+// Implemented by ash to delegate parts of keyboard accelerator handling back
+// to chrome.
+interface AcceleratorController {
+  // Sets the volume controller interface. This lives in chrome because chrome
+  // owns the D-Bus CrasAudioHandler implementation. Also the accessibility
+  // manager in chrome needs to know about volume changes.
+  SetVolumeController(VolumeController controller);
 };