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

ash/test/test_shell_delegate.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/test/test_shell_delegate.h"
 
 #include <limits>
 
 #include "ash/default_accessibility_delegate.h"
 #include "ash/gpu_support_stub.h"
@@ skipping 177 matching lines @@
     ash::ShelfItemDelegate* item_delegate,
     ash::ShelfItem* item) {
   return NULL;
 }
 
 GPUSupport* TestShellDelegate::CreateGPUSupport() {
   // Real GPU support depends on src/content, so just use a stub.
   return new GPUSupportStub;
 }
 
+UserMetricsRecorderProxy* TestShellDelegate::CreateUserMetricsRecorderProxy() {
+  return nullptr;
+}
+
 base::string16 TestShellDelegate::GetProductName() const {
   return base::string16();
 }
 
 void TestShellDelegate::SetMediaCaptureState(MediaCaptureState state) {
 #if defined(OS_CHROMEOS)
   Shell* shell = Shell::GetInstance();
   static_cast<MediaDelegateImpl*>(shell->media_delegate())
       ->set_media_capture_state(state);
   shell->system_tray_notifier()->NotifyMediaCaptureChanged();
 #endif
 }
 
 }  // namespace test
 }  // namespace ash