Visually disable VR Shell back/forward buttons when no history is available

chrome/browser/android/vr_shell/ui_interface.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_ANDROID_VR_SHELL_UI_INTERFACE_H_
 #define CHROME_BROWSER_ANDROID_VR_SHELL_UI_INTERFACE_H_
 
 #include <memory>
 #include <string>
 
@@ skipping 31 matching lines @@
   void SetLoading(bool loading);
   void SetLoadProgress(double progress);
   void InitTabList();
   void AppendToTabList(bool incognito, int id, const base::string16& title);
   void FlushTabList();
   void UpdateTab(bool incognito, int id, const std::string& title);
   void RemoveTab(bool incognito, int id);
   void SetURL(const GURL& url);
   void SetOmniboxSuggestions(std::unique_ptr<base::Value> suggestions);
   void HandleAppButtonClicked();
+  void SetHistoryButtonsEnabled(bool can_go_back, bool can_go_forward);
 
   // Handlers for HTML UI commands and notifications.
   void OnDomContentsLoaded();
   void HandleOmniboxInput(const base::DictionaryValue& input);
 
   void SetUiCommandHandler(UiCommandHandler* handler);
 
  private:
   void FlushUpdates();
   void FlushModeState();
 
   Mode mode_;
   bool fullscreen_ = false;
   UiCommandHandler* handler_;
   bool loaded_ = false;
   base::DictionaryValue updates_;
   std::unique_ptr<base::ListValue> tab_list_;
 
   std::unique_ptr<VrOmnibox> omnibox_;
 
   DISALLOW_COPY_AND_ASSIGN(UiInterface);
 };
 
 }  // namespace vr_shell
 
 #endif  // CHROME_BROWSER_ANDROID_VR_SHELL_UI_INTERFACE_H_