blink: Cleanup core class forward declarations

third_party/WebKit/Source/core/dom/IdleRequestCallback.h

 // Copyright 2015 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 IdleRequestCallback_h
 #define IdleRequestCallback_h
 
 #include "core/dom/ScriptedIdleTaskController.h"
 #include "platform/Timer.h"
 #include "platform/heap/Handle.h"
 
 namespace blink {
 
-class IdleCallbackDeadline;
 
 class IdleRequestCallback
     : public GarbageCollectedFinalized<IdleRequestCallback> {
  public:
   DEFINE_INLINE_VIRTUAL_TRACE() {}
   virtual ~IdleRequestCallback() {}
 
   virtual void handleEvent(IdleDeadline*) = 0;
 };
 
 }  // namespace blink
 
 #endif  // IdleRequestCallback_h