components: Cleanup class/struct fwd declarations

components/sync/engine_impl/events/poll_get_updates_request_event.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 COMPONENTS_SYNC_ENGINE_IMPL_EVENTS_POLL_GET_UPDATES_REQUEST_EVENT_H_
 #define COMPONENTS_SYNC_ENGINE_IMPL_EVENTS_POLL_GET_UPDATES_REQUEST_EVENT_H_
 
 #include <memory>
 #include <string>
 
 #include "base/macros.h"
 #include "base/time/time.h"
 #include "base/values.h"
 #include "components/sync/base/model_type.h"
 #include "components/sync/engine/events/protocol_event.h"
 #include "components/sync/protocol/sync.pb.h"
 
 namespace syncer {
 
-class NudgeTracker;
-
 // An event representing a poll request sent to the server.
 class PollGetUpdatesRequestEvent : public ProtocolEvent {
  public:
   PollGetUpdatesRequestEvent(base::Time timestamp,
                              const sync_pb::ClientToServerMessage& request);
   ~PollGetUpdatesRequestEvent() override;
 
   base::Time GetTimestamp() const override;
   std::string GetType() const override;
   std::string GetDetails() const override;
   std::unique_ptr<base::DictionaryValue> GetProtoMessage() const override;
   std::unique_ptr<ProtocolEvent> Clone() const override;
 
  private:
   const base::Time timestamp_;
   const sync_pb::ClientToServerMessage request_;
 
   DISALLOW_COPY_AND_ASSIGN(PollGetUpdatesRequestEvent);
 };
 
 }  // namespace syncer
 
 #endif  // COMPONENTS_SYNC_ENGINE_IMPL_EVENTS_POLL_GET_UPDATES_REQUEST_EVENT_H_