Move the fpdfsdk/include/pdfwindow/ headers into fpdfsdk/pdfwindow.

fpdfsdk/include/pdfwindow/PWL_Button.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 FPDFSDK_INCLUDE_PDFWINDOW_PWL_BUTTON_H_
-#define FPDFSDK_INCLUDE_PDFWINDOW_PWL_BUTTON_H_
-
-#include "fpdfsdk/include/pdfwindow/PWL_Wnd.h"
-
-class CPWL_Button : public CPWL_Wnd {
- public:
-  CPWL_Button();
-  ~CPWL_Button() override;
-
-  // CPWL_Wnd
-  CFX_ByteString GetClassName() const override;
-  void OnCreate(PWL_CREATEPARAM& cp) override;
-  FX_BOOL OnLButtonDown(const CFX_FloatPoint& point, FX_DWORD nFlag) override;
-  FX_BOOL OnLButtonUp(const CFX_FloatPoint& point, FX_DWORD nFlag) override;
-
- protected:
-  FX_BOOL m_bMouseDown;
-};
-
-#endif  // FPDFSDK_INCLUDE_PDFWINDOW_PWL_BUTTON_H_