Add CanCloseDialog to WebDialogDelegate to allow blocking closing of dialog if needed.

ui/web_dialogs/web_dialog_delegate.cc

 // 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 "ui/web_dialogs/web_dialog_delegate.h"
 
 namespace ui {
 
 std::string WebDialogDelegate::GetDialogName() const {
   return std::string();
 }
 
 void WebDialogDelegate::GetMinimumDialogSize(gfx::Size* size) const {
   GetDialogSize(size);
 }
 
+bool WebDialogDelegate::CanCloseDialog() const {
+  return true;
+}
+
 void WebDialogDelegate::OnDialogCloseFromWebUI(
     const std::string& json_retval) {
   OnDialogClosed(json_retval);
 }
 
 bool WebDialogDelegate::HandleContextMenu(
     const content::ContextMenuParams& params) {
   return false;
 }
 
 bool WebDialogDelegate::HandleOpenURLFromTab(
     content::WebContents* source,
     const content::OpenURLParams& params,
     content::WebContents** out_new_contents) {
   return false;
 }
 
 bool WebDialogDelegate::HandleAddNewContents(
     content::WebContents* source,
     content::WebContents* new_contents,
     WindowOpenDisposition disposition,
     const gfx::Rect& initial_pos,
     bool user_gesture) {
   return false;
 }
 
 }  // namespace ui