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

ui/views/controls/button/label_button.h

Index: ui/views/controls/button/label_button.h
diff --git a/ui/views/controls/button/label_button.h b/ui/views/controls/button/label_button.h
index a7e3a29849e0695a190670d4d730423f8af582a5..90825c6cf80a958bbbc205a329cdcd8adc0e635e 100644
--- a/ui/views/controls/button/label_button.h
+++ b/ui/views/controls/button/label_button.h
@@ -131,6 +131,10 @@ class VIEWS_EXPORT LabelButton : public CustomButton,
   // Resets colors from the NativeTheme, explicitly set colors are unchanged.
   virtual void ResetColorsFromNativeTheme();
 
+  // Changes the visual styling of this button to reflect the state of
+  // |is_default()|.
+  virtual void UpdateStyleForDefaultness();
+
   // Updates the image view to contain the appropriate button state image.
   void UpdateImage();