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

ui/views/controls/button/md_text_button.h

Index: ui/views/controls/button/md_text_button.h
diff --git a/ui/views/controls/button/md_text_button.h b/ui/views/controls/button/md_text_button.h
index 0f6025ae78cf054754bdd67777f5033cbf53028d..902f672f93bf0ef9b1f588296a220c1d573c6e87 100644
--- a/ui/views/controls/button/md_text_button.h
+++ b/ui/views/controls/button/md_text_button.h
@@ -27,6 +27,10 @@ class VIEWS_EXPORT MdTextButton : public LabelButton {
   // in MD mode.
   static LabelButton* CreateStandardButton(ButtonListener* listener,
                                            const base::string16& text);
+  // As above, but only creates an MdTextButton if MD is enabled in the
+  // secondary UI (as opposed to just "top chrome"/"primary" UI).
+  static LabelButton* CreateSecondaryUiButton(ButtonListener* listener,
+                                              const base::string16& text);
   static MdTextButton* CreateMdButton(ButtonListener* listener,
                                       const base::string16& text);
 
@@ -36,6 +40,7 @@ class VIEWS_EXPORT MdTextButton : public LabelButton {
   void OnNativeThemeChanged(const ui::NativeTheme* theme) override;
   SkColor GetInkDropBaseColor() const override;
   void SetText(const base::string16& text) override;
+  void UpdateStyleForDefaultness() override;
 
  private:
   MdTextButton(ButtonListener* listener);