mus: Add comment to PointerWatcher

ui/views/pointer_watcher.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.
 
 #ifndef UI_VIEWS_POINTER_WATCHER_H_
 #define UI_VIEWS_POINTER_WATCHER_H_
 
 #include "ui/views/views_export.h"
 
 namespace ui {
 class MouseEvent;
 class TouchEvent;
 }
 
 namespace views {
 
 // An interface for read-only observation of pointer events (in particular, the
 // events cannot be marked as handled). Only certain event types are supported.
+// NOTE: The event.target is always null, because on mus the target window may
+// be owned by another process.
 class VIEWS_EXPORT PointerWatcher {
  public:
   virtual ~PointerWatcher() {}
 
   virtual void OnMousePressed(const ui::MouseEvent& event) = 0;
   virtual void OnTouchPressed(const ui::TouchEvent& event) = 0;
 };
 
 }  // namespace views
 
 #endif  // UI_VIEWS_POINTER_WATCHER_H_