Don't set a default action for ToolbarActionsBarBubbles.

chrome/browser/ui/views/toolbar/toolbar_actions_bar_bubble_views.h

 // Copyright 2016 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 CHROME_BROWSER_UI_VIEWS_TOOLBAR_TOOLBAR_ACTIONS_BAR_BUBBLE_VIEWS_H_
 #define CHROME_BROWSER_UI_VIEWS_TOOLBAR_TOOLBAR_ACTIONS_BAR_BUBBLE_VIEWS_H_
 
 #include <memory>
 
 #include "base/macros.h"
@@ skipping 25 matching lines @@
   const views::Link* learn_more_button() const { return learn_more_button_; }
 
  private:
   // views::BubbleDialogDelegateView:
   base::string16 GetWindowTitle() const override;
   views::View* CreateExtraView() override;
   bool Cancel() override;
   bool Accept() override;
   bool Close() override;
   int GetDialogButtons() const override;
+  int GetDefaultDialogButton() const override;
   base::string16 GetDialogButtonLabel(ui::DialogButton button) const override;
   void Init() override;
 
   // views::LinkListener:
   void LinkClicked(views::Link* source, int event_flags) override;
 
   std::unique_ptr<ToolbarActionsBarBubbleDelegate> delegate_;
   views::Label* item_list_;
   views::Link* learn_more_button_;
 
   DISALLOW_COPY_AND_ASSIGN(ToolbarActionsBarBubbleViews);
 };
 
 #endif  // CHROME_BROWSER_UI_VIEWS_TOOLBAR_TOOLBAR_ACTIONS_BAR_BUBBLE_VIEWS_H_