Fix missing callback chaining in UsbService.

chrome/browser/extensions/api/usb/usb_api.h

 // Copyright (c) 2012 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 CHROME_BROWSER_EXTENSIONS_API_USB_USB_API_H_
 #define CHROME_BROWSER_EXTENSIONS_API_USB_USB_API_H_
 
 #include <string>
 
 #include "base/memory/ref_counted.h"
@@ skipping 52 matching lines @@
 
   static void SetDeviceForTest(UsbDevice* device);
 
  protected:
   virtual ~UsbFindDevicesFunction();
 
   virtual bool Prepare() OVERRIDE;
   virtual void AsyncWorkStart() OVERRIDE;
 
  private:
+  void OnCompleted();
+
+  scoped_ptr<base::ListValue> result_;
+  std::vector<scoped_refptr<UsbDevice> > devices_;
   scoped_ptr<extensions::api::usb::FindDevices::Params> parameters_;
 };
 
 class UsbCloseDeviceFunction : public UsbAsyncApiFunction {
  public:
   DECLARE_EXTENSION_FUNCTION("usb.closeDevice", USB_CLOSEDEVICE)
 
   UsbCloseDeviceFunction();
 
  protected:
@@ skipping 124 matching lines @@
   virtual bool Prepare() OVERRIDE;
   virtual void AsyncWorkStart() OVERRIDE;
 
  private:
   scoped_ptr<extensions::api::usb::IsochronousTransfer::Params> parameters_;
 };
 
 }  // namespace extensions
 
 #endif  // CHROME_BROWSER_EXTENSIONS_API_USB_USB_API_H_