Disable back button when stack is empty

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

 // 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.
 
 #include "chrome/browser/android/vr_shell/ui_elements/url_bar.h"
 
 #include "base/memory/ptr_util.h"
 #include "chrome/browser/android/vr_shell/textures/url_bar_texture.h"
 
 namespace vr_shell {
@@ skipping 65 matching lines @@
 
 void UrlBar::SetEnabled(bool enabled) {
   enabled_ = enabled;
   set_visible(enabled);
 }
 
 void UrlBar::SetURL(const GURL& gurl) {
   texture_->SetURL(gurl);
 }
 
+void UrlBar::SetHistoryButtonsEnabled(bool can_go_back) {
+  texture_->SetHistoryButtonsEnabled(can_go_back);
+}
+
 void UrlBar::SetSecurityLevel(security_state::SecurityLevel level) {
   texture_->SetSecurityLevel(level);
 }
 
 void UrlBar::SetBackButtonCallback(const base::Callback<void()>& callback) {
   back_button_callback_ = callback;
 }
 
 void UrlBar::OnStateUpdated(const gfx::PointF& position) {
   const bool hovered = texture_->HitsBackButton(position);
   const bool pressed = hovered ? down_ : false;
 
   texture_->SetHovered(hovered);
   texture_->SetPressed(pressed);
   UpdateTexture();
 }
 
 }  // namespace vr_shell