Update MidiDispatcher to use Mojo Permission Service.

content/common/permission_service.mojom

 // Copyright 2014 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 content;
 
 enum PermissionStatus {
   GRANTED,
   DENIED,
   ASK
 };
 
 enum PermissionName {
   GEOLOCATION,
+  MIDI_SYSEX,

[nasko, 2014/11/25 00:30:59]

nit: MIDI_SYS_EX

[mlamouri (slow - plz ping), 2014/11/25 09:48:21]

The current permission name is content::PERMISSION_MIDI_SYSEX. Also, MIDI_SYS_EX
is never used in the code base but MIDI_SYSEX is used some times (a couple
dozen).

I will keep the current name if you don't mind then.

 };
 
 // The Permission service provides permission handling capabilities by exposing
 // methods to check, request, and revoke permissions. It also allows a client to
 // start listening to permission changes.
 interface PermissionService {
   HasPermission(PermissionName permission, string origin)
     => (PermissionStatus status);
   RequestPermission(PermissionName permission, string origin)
     => (PermissionStatus status);
 };