DialogDelegate: allow platforms to prevent Cancel being default

ui/views/style/platform_style.h

 // Copyright 2015 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_STYLE_PLATFORM_STYLE_H_
 #define UI_VIEWS_STYLE_PLATFORM_STYLE_H_
 
 #include <memory>
 
 #include "base/macros.h"
@@ skipping 21 matching lines @@
   // Combobox::STYLE_NORMAL.
   static const int kComboboxNormalArrowPadding;
 
   // Minimum size for platform-styled buttons (Button::STYLE_BUTTON).
   static const int kMinLabelButtonWidth;
   static const int kMinLabelButtonHeight;
 
   // Whether dialog-default buttons are given a bold font style.
   static const bool kDefaultLabelButtonHasBoldFont;
 
+  // Whether the default button for a dialog can be the Cancel button.
+  static const bool kDialogDefaultButtonCanBeCancel;
+
   // Whether dragging vertically above or below a textfield's bounds selects to
   // the left or right end of the text from the cursor, respectively.
   static const bool kTextfieldDragVerticallyDragsToEnd;
 
   // The menu button's action to show the menu.
   static const CustomButton::NotifyAction kMenuNotifyActivationAction;
 
   // Creates an ImageSkia containing the image to use for the combobox arrow.
   // The |is_enabled| argument is true if the control the arrow is for is
   // enabled, and false if the control is disabled. The |style| argument is the
@@ skipping 30 matching lines @@
   static std::unique_ptr<Border> CreateThemedLabelButtonBorder(
       LabelButton* button);
 
  private:
   DISALLOW_IMPLICIT_CONSTRUCTORS(PlatformStyle);
 };
 
 }  // namespace views
 
 #endif  // UI_VIEWS_STYLE_PLATFORM_STYLE_H_