Provide hook to abort dialog cancellation, disable cancellation during install.

ui/views/window/dialog_client_view.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/views/window/dialog_client_view.h"
 
 #include "build/build_config.h"
 
 #if defined(OS_WIN)
 #include <windows.h>
@@ skipping 241 matching lines @@
 ///////////////////////////////////////////////////////////////////////////////
 // DialogClientView, ClientView overrides:
 
 bool DialogClientView::CanClose() {
   if (notified_delegate_)
     return true;
 
   DialogDelegate* dd = GetDialogDelegate();
   int buttons = dd->GetDialogButtons();
   bool close = true;
-  if (buttons & ui::DIALOG_BUTTON_CANCEL)
+  if ((buttons & ui::DIALOG_BUTTON_CANCEL) ||
+      (buttons == ui::DIALOG_BUTTON_NONE))
     close = dd->Cancel();
   else if (buttons & ui::DIALOG_BUTTON_OK)
     close = dd->Accept(true);
   notified_delegate_ = close;
   return close;
 }
 
 void DialogClientView::WidgetClosing() {
   if (listening_to_focus_) {
     DCHECK(saved_focus_manager_);
@@ skipping 249 matching lines @@
   }
   saved_focus_manager_ = focus_manager;
   // Listen for focus change events so we can update the default button.
   if (focus_manager) {
     focus_manager->AddFocusChangeListener(this);
     listening_to_focus_ = true;
   }
 }
 
 }  // namespace views