Remove include only fwl.h and IWYU.

xfa/include/fwl/core/fwl_thread.h

 // Copyright 2014 PDFium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 // Original code copyright 2014 Foxit Software Inc. http://www.foxitsoftware.com
 
 #ifndef XFA_INCLUDE_FWL_CORE_FWL_THREAD_H_
 #define XFA_INCLUDE_FWL_CORE_FWL_THREAD_H_
 
 // The FWL thread/app code contains three parallel inheritance hierarchies,
 // which reference each other via pointers as follows:
 //
 //                                           m_pImpl
 //      (nonesuch)              IFWL_Thread ----------> CFWL_ThreadImp
 //                                   |      <----------      |
 //                                   A       m_pIface        A
 //                                   |                       |
 //      (nonesuch)              IFWL_NoteThread         CFWL_NoteThreadImp
 //                                   |                       |
 //                                   A                       A
 //                m_pIface           |                       |
 //      CFWL_App --------------> IFWL_App                CFWL_AppImp
 //
 
+#include "xfa/include/fwl/core/fwl_error.h"
+
 class CFWL_ThreadImp;
 class IFWL_NoteDriver;
 
 typedef struct FWL_HTHREAD_ { void* pData; } * FWL_HTHREAD;
 
 class IFWL_Thread {
  public:
   // These call into polymorphic methods in the impl; no need to override.
   void Release();
   FWL_ERR Run(FWL_HTHREAD hThread);
 
   CFWL_ThreadImp* GetImpl() const { return m_pImpl; }
   void SetImpl(CFWL_ThreadImp* pImpl) { m_pImpl = pImpl; }
 
  protected:
   virtual ~IFWL_Thread() {}
 
  private:
   CFWL_ThreadImp* m_pImpl;
 };
 
 class IFWL_NoteThread : public IFWL_Thread {
  public:
   IFWL_NoteDriver* GetNoteDriver();
 };
 
 #endif  // XFA_INCLUDE_FWL_CORE_FWL_THREAD_H_