mus: Add EventObserver to allow passively listening to UI events

components/mus/ws/event_dispatcher_delegate.h

 // Copyright 2015 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 COMPONENTS_MUS_WS_EVENT_DISPATCHER_DELEGATE_H_
 #define COMPONENTS_MUS_WS_EVENT_DISPATCHER_DELEGATE_H_
 
 #include <stdint.h>
 
 namespace ui {
@@ skipping 23 matching lines @@
   // holding capture. The delegate should not change native display capture.
   // ReleaseNativeCapture is invoked if appropriate.
   virtual void OnServerWindowCaptureLost(ServerWindow* window) = 0;
 
   // |in_nonclient_area| is true if the event occurred in the non-client area.
   virtual void DispatchInputEventToWindow(ServerWindow* target,
                                           bool in_nonclient_area,
                                           const ui::Event& event,
                                           Accelerator* accelerator) = 0;
 
+  // Called when event dispatch could not find a target.

[sky, 2016/04/22 20:18:48]

Clarify that OnAccelerator may still be called.

[James Cook, 2016/04/22 21:25:08]

Done.

+  virtual void OnEventTargetNotFound(const ui::Event& event) = 0;
+
  protected:
   virtual ~EventDispatcherDelegate() {}
 };
 
 }  // namespace ws
 }  // namespace mus
 
 #endif  // COMPONENTS_MUS_WS_EVENT_DISPATCHER_DELEGATE_H_