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

chrome/browser/android/vr_shell/vr_shell.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_VR_SHELL_H_
 #define CHROME_BROWSER_ANDROID_VR_SHELL_VR_SHELL_H_
 
 #include <jni.h>
 
 #include <memory>
@@ skipping 98 matching lines @@
                     jstring jtitle);
   void OnTabRemoved(JNIEnv* env,
                     const base::android::JavaParamRef<jobject>& obj,
                     jboolean incognito,
                     jint id);
   base::android::ScopedJavaGlobalRef<jobject> TakeContentSurface(
       JNIEnv* env,
       const base::android::JavaParamRef<jobject>& obj);
   void RestoreContentSurface(JNIEnv* env,
                              const base::android::JavaParamRef<jobject>& obj);
+  void SetHistoryButtonsEnabled(JNIEnv* env,
+                                const base::android::JavaParamRef<jobject>& obj,
+                                jboolean can_go_back,
+                                jboolean can_go_forward);
 
   void ContentWebContentsDestroyed();
   // Called when our WebContents have been hidden. Usually a sign that something
   // like another tab placed in front of it.
   void ContentWasHidden();
   void ContentWasShown();
 
   // html/js UI hooks.
   static base::WeakPtr<VrShell> GetWeakPtr(
       const content::WebContents* web_contents);
@@ skipping 109 matching lines @@
   base::WeakPtrFactory<VrShell> weak_ptr_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(VrShell);
 };
 
 bool RegisterVrShell(JNIEnv* env);
 
 }  // namespace vr_shell
 
 #endif  // CHROME_BROWSER_ANDROID_VR_SHELL_VR_SHELL_H_