Make MdTextButton a LabelButton, use it in the DL shelf.

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 eee771e79222b726ee2fa2dfb309279675a0893b..500e5aa94f3f15da03ff00c688eb41385396ab31 100644
--- a/ui/views/controls/button/label_button.h
+++ b/ui/views/controls/button/label_button.h
@@ -45,7 +45,7 @@ class VIEWS_EXPORT LabelButton : public CustomButton,
 
   // Gets or sets the text shown on the button.
   const base::string16& GetText() const;
-  void SetText(const base::string16& text);
+  virtual void SetText(const base::string16& text);
 
   // Sets the text color shown for the specified button |for_state| to |color|.
   void SetTextColor(ButtonState for_state, SkColor color);
@@ -147,6 +147,8 @@ class VIEWS_EXPORT LabelButton : public CustomButton,
   FRIEND_TEST_ALL_PREFIXES(LabelButtonTest, FontList);
   FRIEND_TEST_ALL_PREFIXES(LabelButtonTest, ButtonStyleIsDefaultSize);
 
+  void SetTextInternal(const base::string16& text);
+
   // View:
   void ChildPreferredSizeChanged(View* child) override;