Change IFWL_Widget to store a single delegate.

xfa/fwl/core/ifwl_listbox.cpp

 // 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
 
 #include "xfa/fwl/core/ifwl_listbox.h"
 
 #include "third_party/base/ptr_util.h"
 #include "xfa/fde/tto/fde_textout.h"
@@ skipping 1044 matching lines @@
 void CFWL_ListBoxImpDelegate::OnLButtonUp(CFWL_MsgMouse* pMsg) {
   if (m_pOwner->m_bLButtonDown) {
     m_pOwner->m_bLButtonDown = FALSE;
     m_pOwner->SetGrab(FALSE);
     DispatchSelChangedEv();
   }
 }
 
 void CFWL_ListBoxImpDelegate::OnMouseWheel(CFWL_MsgMouseWheel* pMsg) {
   if (m_pOwner->IsShowScrollBar(TRUE))
-    m_pOwner->m_pVertScrollBar->GetCurrentDelegate()->OnProcessMessage(pMsg);
+    m_pOwner->m_pVertScrollBar->GetDelegate()->OnProcessMessage(pMsg);
 }
 
 void CFWL_ListBoxImpDelegate::OnKeyDown(CFWL_MsgKey* pMsg) {
   uint32_t dwKeyCode = pMsg->m_dwKeyCode;
   switch (dwKeyCode) {
     case FWL_VKEY_Tab:
     case FWL_VKEY_Up:
     case FWL_VKEY_Down:
     case FWL_VKEY_Home:
     case FWL_VKEY_End: {
@@ skipping 99 matching lines @@
     m_pOwner->Repaint(&m_pOwner->m_rtClient);
   }
   return TRUE;
 }
 
 void CFWL_ListBoxImpDelegate::DispatchSelChangedEv() {
   CFWL_EvtLtbSelChanged ev;
   ev.m_pSrcTarget = m_pOwner;
   m_pOwner->DispatchEvent(&ev);
 }