mus: Add PointerWatcher for passively observing mouse and touch events

ash/pointer_watcher_delegate.h

+// 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.
+
+#include "ash/ash_export.h"
+
+namespace views {
+class PointerWatcher;
+}
+
+namespace ash {
+
+// Allows different implementations of PointerWatcher in mus and non-mus ash.
+class ASH_EXPORT PointerWatcherDelegate {
+ public:
+  virtual ~PointerWatcherDelegate() {}
+
+  virtual void AddPointerWatcher(views::PointerWatcher* watcher) = 0;
+  virtual void RemovePointerWatcher(views::PointerWatcher* watcher) = 0;
+};
+
+}  // namespace ash