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

chrome/browser/android/vr_shell/ui_interface.cc

 // 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.
 
 #include "chrome/browser/android/vr_shell/ui_interface.h"
 
 #include <memory>
 #include <utility>
 
 #include "base/memory/ptr_util.h"
@@ skipping 97 matching lines @@
 
   updates_.Set("url", std::move(details));
   FlushUpdates();
 }
 
 void UiInterface::HandleAppButtonClicked() {
   updates_.SetBoolean("appButtonClicked", true);
   FlushUpdates();
 }
 
+void UiInterface::SetHistoryButtonsEnabled(bool can_go_back,
+                                           bool can_go_forward) {
+  updates_.SetBoolean("canGoBack", can_go_back);
+  updates_.SetBoolean("canGoForward", can_go_forward);
+  FlushUpdates();
+}
+
 void UiInterface::OnDomContentsLoaded() {
   loaded_ = true;
 #if defined(ENABLE_VR_SHELL_UI_DEV)
   updates_.SetBoolean("enableReloadUi", true);
 #endif
   FlushUpdates();
 }
 
 void UiInterface::SetUiCommandHandler(UiCommandHandler* handler) {
   handler_ = handler;
 }
 
 void UiInterface::FlushUpdates() {
   if (loaded_ && handler_) {
     handler_->SendCommandToUi(updates_);
     updates_.Clear();
   }
 }
 
 }  // namespace vr_shell