Add Mojo IPC based input-device service.

components/mus/public/interfaces/input_devices/input_devices.mojom

Index: components/mus/public/interfaces/input_devices/input_devices.mojom
diff --git a/components/mus/public/interfaces/input_devices/input_devices.mojom b/components/mus/public/interfaces/input_devices/input_devices.mojom
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index 0000000000000000000000000000000000000000..fa738b98d962e1dc76843aae5c8ecee1f62fae94
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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 input_device.mojom;
+
+import "ui/events/devices/mojo/input_devices.mojom";
+
+// Receives updates about changes to input-devices. See InputDeviceServer for
+// the expected behaviour.
+interface InputDeviceObserverMojo {
+  OnKeyboardDeviceConfigurationChanged(array<ui.mojom.InputDevice> devices);
+  OnTouchscreenDeviceConfigurationChanged(
+      array<ui.mojom.TouchscreenDevice> devices);
+  OnMouseDeviceConfigurationChanged(array<ui.mojom.InputDevice> devices);
+  OnTouchpadDeviceConfigurationChanged(array<ui.mojom.InputDevice> devices);
+
+  // Will be called after the observer has received all of the input-device
+  // lists. Until this is called the observer does not know if an empty list is
+  // empty or has yet to be received.
+  OnDeviceListsComplete();

[sadrul, 2016/06/08 16:31:09]

OK. So, it sounds like once the observer registers itself, it's can get upto 5
IPC messages (4 for the device-type lists, and then this
OnDeviceListsComplete()). Can these be merged into one, e.g. can OnDeviceLists()
include the various lists?

[kylechar, 2016/06/08 18:13:16]

Yeah, that's definitely possible. Not sure it will make a difference in practice
but I've made it such that OnDeviceListsComplete() includes all of the lists and
no updates are sent before it.

+};
+
+// When the implementation receives an updated input-device list it will send
+// the new list to all registered observers. Additionally, if receives
+// OnDeviceListsComplete() it will send that message to all registered
+// observers as well.
+interface InputDeviceServer {
+  // Adds an InputDeviceObserverMojo in another process as an observer to get
+  // notified of changes to input-devices over Mojo IPC. The implementation
+  // should send available list of input-devices to a new observer when it 
+  // connects.
+  //
+  // The implementation should also call OnDeviceListsComplete() after sending
+  // all input-device lists to an observer, assuming it has received all of the
+  // input-device lists itself. Otherwise the implementation will wait until it
+  // receives OnDeviceListsComplete() to send the message to observers.
+  AddObserver(InputDeviceObserverMojo observer);
+};