components: Cleanup class/struct fwd declarations

components/sync/engine_impl/sync_cycle_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_SYNC_CYCLE_EVENT_H_
 #define COMPONENTS_SYNC_ENGINE_IMPL_SYNC_CYCLE_EVENT_H_
 
 #include "components/sync/engine/cycle/sync_cycle_snapshot.h"
 
 namespace syncer {
 
-struct SyncProtocolError;
-
 struct SyncCycleEvent {
   enum EventCause {
     ////////////////////////////////////////////////////////////////
     // Sent on entry of Syncer state machine
     SYNC_CYCLE_BEGIN,
 
     // Sent any time progress is made during a sync cycle.
     STATUS_CHANGED,
 
     // We have reached the SYNCER_END state in the main sync loop.
     SYNC_CYCLE_ENDED,
   };
 
   explicit SyncCycleEvent(EventCause cause);
   ~SyncCycleEvent();
 
   EventCause what_happened;
 
   // The last cycle used for syncing.
   SyncCycleSnapshot snapshot;
 };
 
 }  // namespace syncer
 
 #endif  // COMPONENTS_SYNC_ENGINE_IMPL_SYNC_CYCLE_EVENT_H_