Disable back button when stack is empty

chrome/browser/android/vr_shell/ui_scene_manager.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_scene_manager.h"
 
 #include "base/callback.h"
 #include "base/memory/ptr_util.h"
 #include "chrome/browser/android/vr_shell/textures/close_button_texture.h"
 #include "chrome/browser/android/vr_shell/textures/ui_texture.h"
@@ skipping 440 matching lines @@
 }
 
 void UiSceneManager::SetIsExiting() {
   if (scene_->is_exiting())
     return;
   scene_->set_is_exiting();
   ConfigureScene();
 }
 
 void UiSceneManager::SetHistoryButtonsEnabled(bool can_go_back,
-                                              bool can_go_forward) {}
+                                              bool can_go_forward) {
+  url_bar_->SetHistoryButtonsEnabled(can_go_back);
+}
 
 void UiSceneManager::OnCloseButtonClicked() {
   browser_->ExitCct();
 }
 
 void UiSceneManager::OnUnsupportedMode(UiUnsupportedMode mode) {
   browser_->OnUnsupportedMode(mode);
 }
 
 int UiSceneManager::AllocateId() {
   return next_available_id_++;
 }
 
 ColorScheme::Mode UiSceneManager::mode() const {
   if (incognito_)
     return ColorScheme::kModeIncognito;
   if (fullscreen_)
     return ColorScheme::kModeFullscreen;
   return ColorScheme::kModeNormal;
 }
 
 const ColorScheme& UiSceneManager::color_scheme() const {
   return ColorScheme::GetColorScheme(mode());
 }
 
 }  // namespace vr_shell