DialogDelegate: allow platforms to prevent Cancel being default

ui/views/style/platform_style_mac.mm

 // 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.
 
 #include "ui/views/style/platform_style.h"
 
 #include "base/memory/ptr_util.h"
 #include "ui/base/resource/resource_bundle.h"
 #include "ui/gfx/paint_vector_icon.h"
 #include "ui/gfx/vector_icons.h"
 #include "ui/resources/grit/ui_resources.h"
 #include "ui/views/controls/button/label_button.h"
 #include "ui/views/controls/button/label_button_border.h"
 #include "ui/views/controls/focusable_rounded_border_mac.h"
 #import "ui/views/controls/scrollbar/cocoa_scroll_bar.h"
 #include "ui/views/style/mac/combobox_background_mac.h"
 #include "ui/views/style/mac/dialog_button_border_mac.h"
 
 namespace views {
 
 const int PlatformStyle::kComboboxNormalArrowPadding = 0;
 const int PlatformStyle::kMinLabelButtonWidth = 32;
 const int PlatformStyle::kMinLabelButtonHeight = 30;
 const bool PlatformStyle::kDefaultLabelButtonHasBoldFont = false;
+const bool PlatformStyle::kDialogDefaultButtonCanBeCancel = false;
 const bool PlatformStyle::kTextfieldDragVerticallyDragsToEnd = true;
 
 const CustomButton::NotifyAction PlatformStyle::kMenuNotifyActivationAction =
     CustomButton::NOTIFY_ON_PRESS;
 
 // static
 gfx::ImageSkia PlatformStyle::CreateComboboxArrow(bool is_enabled,
                                                   Combobox::Style style) {
   // TODO(ellyjones): IDR_MENU_DROPARROW is a cross-platform image that doesn't
   // look right on Mac. See https://crbug.com/384071.
@@ skipping 51 matching lines @@
 void PlatformStyle::ApplyLabelButtonTextStyle(
     views::Label* label,
     ButtonColorByState* color_by_state) {
   const ui::NativeTheme* theme = label->GetNativeTheme();
   ButtonColorByState& colors = *color_by_state;
   colors[Button::STATE_PRESSED] =
       theme->GetSystemColor(ui::NativeTheme::kColorId_ButtonHighlightColor);
 }
 
 }  // namespace views