DialogDelegate: allow platforms to prevent Cancel being default

ui/views/window/dialog_client_view_unittest.cc

Index: ui/views/window/dialog_client_view_unittest.cc
diff --git a/ui/views/window/dialog_client_view_unittest.cc b/ui/views/window/dialog_client_view_unittest.cc
index c483d254af32ed419283c1ab4ff5fa0f00d075c0..dfe45b562370dae7cf8f7a9d8dd89f83857c7647 100644
--- a/ui/views/window/dialog_client_view_unittest.cc
+++ b/ui/views/window/dialog_client_view_unittest.cc
@@ -7,6 +7,7 @@
 #include "build/build_config.h"
 #include "ui/base/ui_base_types.h"
 #include "ui/views/controls/button/label_button.h"
+#include "ui/views/style/platform_style.h"
 #include "ui/views/test/test_views.h"
 #include "ui/views/test/views_test_base.h"
 #include "ui/views/widget/widget.h"
@@ -148,7 +149,8 @@ TEST_F(DialogClientViewTest, UpdateButtons) {
   // Reset with just a cancel button.
   SetDialogButtons(ui::DIALOG_BUTTON_CANCEL);
   EXPECT_EQ(NULL, client_view()->ok_button());
-  EXPECT_TRUE(client_view()->cancel_button()->is_default());
+  EXPECT_EQ(client_view()->cancel_button()->is_default(),
+            PlatformStyle::kDialogDefaultButtonCanBeCancel);
   EXPECT_EQ(GetUpdatedClientBounds().height(), height_with_buttons);
 }