Change IFWL_Widget to store a single delegate.

xfa/fwl/core/ifwl_combolist.cpp

 // Copyright 2016 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_combolist.h"
 
 #include "third_party/base/ptr_util.h"
 #include "xfa/fwl/core/ifwl_combobox.h"
@@ skipping 94 matching lines @@
       dwHashCode == CFWL_MessageType::KillFocus) {
     OnDropListFocusChanged(pMessage, dwHashCode == CFWL_MessageType::SetFocus);
   } else if (dwHashCode == CFWL_MessageType::Mouse) {
     CFWL_MsgMouse* pMsg = static_cast<CFWL_MsgMouse*>(pMessage);
     if (m_pOwner->IsShowScrollBar(TRUE) && m_pOwner->m_pVertScrollBar) {
       CFX_RectF rect;
       m_pOwner->m_pVertScrollBar->GetWidgetRect(rect);
       if (rect.Contains(pMsg->m_fx, pMsg->m_fy)) {
         pMsg->m_fx -= rect.left;
         pMsg->m_fy -= rect.top;
-        m_pOwner->m_pVertScrollBar->GetCurrentDelegate()->OnProcessMessage(
-            pMsg);
+        m_pOwner->m_pVertScrollBar->GetDelegate()->OnProcessMessage(pMsg);
         return;
       }
     }
     switch (pMsg->m_dwCmd) {
       case FWL_MouseCommand::Move: {
         backDefault = FALSE;
         OnDropListMouseMove(pMsg);
         break;
       }
       case FWL_MouseCommand::LeftButtonDown: {
@@ skipping 142 matching lines @@
       CFX_RectF rtInvalidate;
       rtInvalidate.Set(0, 0, m_pOwner->m_pProperties->m_rtWidget.width,
                        m_pOwner->m_pProperties->m_rtWidget.height);
       m_pOwner->Repaint(&rtInvalidate);
       break;
     }
     default:
       break;
   }
 }