ppapi: Cleanup class/struct forward declarations

ppapi/cpp/network_monitor.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 PPAPI_CPP_NETWORK_MONITOR_H_
 #define PPAPI_CPP_NETWORK_MONITOR_H_
 
 #include <stdint.h>
 
 #include "ppapi/cpp/instance_handle.h"
 #include "ppapi/cpp/resource.h"
 
 namespace pp {
 
-class Instance;
 class NetworkList;
 
 template <typename T> class CompletionCallbackWithOutput;
 
 class NetworkMonitor : public Resource {
  public:
   explicit NetworkMonitor(const InstanceHandle& instance);
 
   int32_t UpdateNetworkList(
       const CompletionCallbackWithOutput<NetworkList>& callback);
 
   // Returns true if the required interface is available.
   static bool IsAvailable();
 };
 
 }  // namespace pp
 
 #endif  // PPAPI_CPP_NETWORK_MONITOR_H_