Factor out uses of the WebContentsModalDialog interface from platform-independent code

chrome/browser/ui/cocoa/tab_modal_confirm_dialog_mac.mm

 // Copyright (c) 2012 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.
 
 #include "chrome/browser/ui/cocoa/tab_modal_confirm_dialog_mac.h"
 
 #include "base/memory/scoped_nsobject.h"
 #include "chrome/browser/ui/browser_dialogs.h"
 #import "chrome/browser/ui/cocoa/constrained_window/constrained_window_alert.h"
 #import "chrome/browser/ui/cocoa/constrained_window/constrained_window_custom_sheet.h"
@@ skipping 59 matching lines @@
                      target:bridge_
                      action:@selector(onCancelButton:)];
   [[alert_ closeButton] setTarget:bridge_];
   [[alert_ closeButton] setAction:@selector(onCancelButton:)];
   [alert_ layout];
 
   scoped_nsobject<CustomConstrainedWindowSheet> sheet(
       [[CustomConstrainedWindowSheet alloc]
           initWithCustomWindow:[alert_ window]]);
   window_.reset(new ConstrainedWindowMac(this, web_contents, sheet));
-  delegate->set_window(window_.get());
+  delegate_->set_close_delegate(this);
 }
 
 TabModalConfirmDialogMac::~TabModalConfirmDialogMac() {
 }
 
 void TabModalConfirmDialogMac::AcceptTabModalDialog() {
   delegate_->Accept();
 }
 
 void TabModalConfirmDialogMac::CancelTabModalDialog() {
   delegate_->Cancel();
 }
 
+void TabModalConfirmDialogMac::CloseDialog() {
+  window_->CloseWebContentsModalDialog();
+}
+
 void TabModalConfirmDialogMac::OnConstrainedWindowClosed(
     ConstrainedWindowMac* window) {
   delete this;
 }