Introduce new flag to control usage of MD in secondary (not top-chrome)

ui/views/controls/button/label_button.cc

Index: ui/views/controls/button/label_button.cc
diff --git a/ui/views/controls/button/label_button.cc b/ui/views/controls/button/label_button.cc
index 08c0a033f4ba872f7ff47cc3ffaffc8f829f1346..d57ea023f269d7d78590dfa85ef012d3ddca71ee 100644
--- a/ui/views/controls/button/label_button.cc
+++ b/ui/views/controls/button/label_button.cc
@@ -201,10 +201,7 @@ void LabelButton::SetIsDefault(bool is_default) {
   ui::Accelerator accel(ui::VKEY_RETURN, ui::EF_NONE);
   is_default_ ? AddAccelerator(accel) : RemoveAccelerator(accel);
 
-  const bool bold = PlatformStyle::kDefaultLabelButtonHasBoldFont && is_default;
-  label_->SetFontList(bold ? cached_bold_font_list_ : cached_normal_font_list_);
-  InvalidateLayout();
-  ResetLabelEnabledColor();
+  UpdateStyleForDefaultness();
 }
 
 void LabelButton::SetStyle(ButtonStyle style) {
@@ -493,6 +490,14 @@ void LabelButton::ResetColorsFromNativeTheme() {
   }
 }
 
+void LabelButton::UpdateStyleForDefaultness() {
+  const bool bold =
+      PlatformStyle::kDefaultLabelButtonHasBoldFont && is_default_;
+  label_->SetFontList(bold ? cached_bold_font_list_ : cached_normal_font_list_);
+  InvalidateLayout();
+  ResetLabelEnabledColor();
+}
+
 void LabelButton::UpdateImage() {
   image_->SetImage(GetImage(state()));
   ResetCachedPreferredSize();