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

ui/views/controls/button/md_text_button.h

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef UI_VIEWS_CONTROLS_BUTTON_MD_TEXT_BUTTON_H_
 #define UI_VIEWS_CONTROLS_BUTTON_MD_TEXT_BUTTON_H_
 
 #include <memory>
 
 #include "ui/views/animation/button_ink_drop_delegate.h"
 #include "ui/views/controls/button/label_button.h"
 
 namespace views {
 
 // A button class that implements the Material Design text button spec.
 class VIEWS_EXPORT MdTextButton : public LabelButton {
  public:
   // Describes the presentation of a button. A stronger call to action draws
   // more attention.
   enum CallToAction {
     NO_CALL_TO_ACTION,  // Default.
     WEAK_CALL_TO_ACTION,
     STRONG_CALL_TO_ACTION,
   };
 
   // Creates a normal STYLE_BUTTON LabelButton in pre-MD, or an MdTextButton
   // 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);
 
   void SetCallToAction(CallToAction cta);
 
   // 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);
   ~MdTextButton() override;
 
   void UpdateColorsFromNativeTheme();
 
   ButtonInkDropDelegate ink_drop_delegate_;
 
   // The call to action style for this button.
   CallToAction cta_;
 
   DISALLOW_COPY_AND_ASSIGN(MdTextButton);
 };
 
 }  // namespace views
 
 #endif  // UI_VIEWS_CONTROLS_BUTTON_MD_TEXT_BUTTON_H_