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

ui/views/window/dialog_delegate.h

 // 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.
 
 #ifndef UI_VIEWS_WINDOW_DIALOG_DELEGATE_H_
 #define UI_VIEWS_WINDOW_DIALOG_DELEGATE_H_
 
 #include "base/compiler_specific.h"
 #include "base/string16.h"
 #include "ui/base/accessibility/accessibility_types.h"
@@ skipping 56 matching lines @@
   // row as the OK and CANCEL buttons but flush to the left and extending
   // up to the buttons.
   virtual View* GetExtraView();
 
   // Returns whether the height of the extra view should be at least as tall as
   // the buttons. The default (false) is to give the extra view its preferred
   // height. By returning true the height becomes
   // max(extra_view preferred height, buttons preferred height).
   virtual bool GetSizeExtraViewHeightToButtons();
 
-  // For Dialog boxes, if there is a "Cancel" button, this is called when the
-  // user presses the "Cancel" button or the Close button on the window or
-  // in the system menu, or presses the Esc key. This function should return
-  // true if the window can be closed after it returns, or false if it must
-  // remain open.
+  // For Dialog boxes, if there is a "Cancel" button or no dialog button at all,
+  // this is called when the user presses the "Cancel" button or the Close
+  // button on the window or in the system menu, or presses the Esc key.
+  // This function should return true if the window can be closed after it
+  // returns, or false if it must remain open.
   virtual bool Cancel();
 
   // For Dialog boxes, this is called when the user presses the "OK" button,
   // or the Enter key.  Can also be called on Esc key or close button
   // presses if there is no "Cancel" button.  This function should return
   // true if the window can be closed after it returns, or false if it must
   // remain open.  If |window_closing| is true, it means that this handler is
   // being called because the window is being closed (e.g.  by Window::Close)
   // and there is no Cancel handler, so Accept is being called instead.
   virtual bool Accept(bool window_closing);
@@ skipping 29 matching lines @@
   virtual Widget* GetWidget() OVERRIDE;
   virtual const Widget* GetWidget() const OVERRIDE;
 
  private:
   DISALLOW_COPY_AND_ASSIGN(DialogDelegateView);
 };
 
 }  // namespace views
 
 #endif  // UI_VIEWS_WINDOW_DIALOG_DELEGATE_H_