DialogDelegate: allow platforms to prevent Cancel being default

ui/views/style/platform_style.h

Index: ui/views/style/platform_style.h
diff --git a/ui/views/style/platform_style.h b/ui/views/style/platform_style.h
index bca38df85395b612d7e6671a1c37205b8ff4dc29..542c8c301800b6d18a85fe93daee2441b238a69b 100644
--- a/ui/views/style/platform_style.h
+++ b/ui/views/style/platform_style.h
@@ -39,6 +39,9 @@ class VIEWS_EXPORT PlatformStyle {
   // 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;