Add a UsbService::Observer function for cleanup actions.

device/usb/usb_service.h

 // 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.
 
 #ifndef DEVICE_USB_USB_SERVICE_H_
 #define DEVICE_USB_USB_SERVICE_H_
 
 #include <vector>
 
 #include "base/memory/ref_counted.h"
@@ skipping 17 matching lines @@
 // All functions on this object must be called from a thread with a
 // MessageLoopForIO (for example, BrowserThread::FILE).
 class UsbService : public base::NonThreadSafe {
  public:
   class Observer {
    public:
     // These events are delivered from the thread on which the UsbService object
     // was created.
     virtual void OnDeviceAdded(scoped_refptr<UsbDevice> device);
     virtual void OnDeviceRemoved(scoped_refptr<UsbDevice> device);
+    // For observers that need to process device removal after others have run.
+    // Should not depend on any other service's knowledge of connected devices.
+    virtual void OnDeviceRemovedCleanup(scoped_refptr<UsbDevice> device);
   };
 
   // The UI task runner reference is used to talk to the PermissionBrokerClient
   // on ChromeOS (UI thread). Returns NULL when initialization fails.
   static UsbService* GetInstance(
       scoped_refptr<base::SingleThreadTaskRunner> ui_task_runner);
 
   static void SetInstanceForTest(UsbService* instance);
 
   virtual scoped_refptr<UsbDevice> GetDeviceById(uint32 unique_id) = 0;
@@ skipping 17 matching lines @@
 
  private:
   class Destroyer;
 
   DISALLOW_COPY_AND_ASSIGN(UsbService);
 };
 
 }  // namespace device
 
 #endif  // DEVICE_USB_USB_SERVICE_H_