Added the Ash.Tab.TimeBetweenSwitchToExistingTabUserActions histogram to track time between tab swit

ash/shell/shell_delegate_impl.cc

 // Copyright (c) 2012 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 "ash/shell/shell_delegate_impl.h"
 
 #include "ash/accessibility_delegate.h"
 #include "ash/default_accessibility_delegate.h"
 #include "ash/default_user_wallpaper_delegate.h"
 #include "ash/gpu_support_stub.h"
@@ skipping 248 matching lines @@
     ash::ShelfItemDelegate* item_delegate,
     ash::ShelfItem* item) {
   return new ContextMenu(root);
 }
 
 GPUSupport* ShellDelegateImpl::CreateGPUSupport() {
   // Real GPU support depends on src/content, so just use a stub.
   return new GPUSupportStub;
 }
 
+UserMetricsRecorderProxy* ShellDelegateImpl::CreateUserMetricsRecorderProxy() {
+  return nullptr;
+}
+
 base::string16 ShellDelegateImpl::GetProductName() const {
   return base::string16();
 }
 
 }  // namespace shell
 }  // namespace ash