Disable back button when stack is empty

chrome/browser/android/vr_shell/ui_elements/url_bar.h

 // Copyright 2017 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_ANDROID_VR_SHELL_UI_ELEMENTS_URL_BAR_H_
 #define CHROME_BROWSER_ANDROID_VR_SHELL_UI_ELEMENTS_URL_BAR_H_
 
 #include <memory>
 
 #include "base/callback.h"
@@ skipping 17 matching lines @@
   void OnHoverEnter(const gfx::PointF& position) override;
   void OnHoverLeave() override;
   void OnMove(const gfx::PointF& position) override;
   void OnButtonDown(const gfx::PointF& position) override;
   void OnButtonUp(const gfx::PointF& position) override;
   bool HitTest(const gfx::PointF& point) const override;
 
   void OnBeginFrame(const base::TimeTicks& begin_frame_time) override;
   void SetEnabled(bool enabled) override;
 
+  void SetHistoryButtonsEnabled(bool can_go_back);
   void SetURL(const GURL& gurl);
   void SetSecurityLevel(security_state::SecurityLevel level);
   void SetBackButtonCallback(const base::Callback<void()>& callback);
 
  private:
   void UpdateTexture() override;
   UiTexture* GetTexture() const override;
   void OnStateUpdated(const gfx::PointF& position);
 
   std::unique_ptr<UrlBarTexture> texture_;
   base::Callback<void()> back_button_callback_;
   bool enabled_ = false;
   bool down_ = false;
   base::TimeTicks last_begin_frame_time_;
   base::TimeTicks last_update_time_;
 
   DISALLOW_COPY_AND_ASSIGN(UrlBar);
 };
 
 }  // namespace vr_shell
 
 #endif  // CHROME_BROWSER_ANDROID_VR_SHELL_UI_ELEMENTS_URL_BAR_H_