Add missing #include around sequenced_worker_pool.h in //chrome

chrome/browser/metrics/variations/chrome_variations_service_client.cc

 // Copyright 2015 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/metrics/variations/chrome_variations_service_client.h"
 
 #include "base/bind.h"
+#include "base/threading/sequenced_worker_pool.h"
 #include "build/build_config.h"
 #include "chrome/browser/browser_process.h"
 #include "chrome/common/channel_info.h"
 #include "components/version_info/version_info.h"
 #include "content/public/browser/browser_thread.h"
 
 #if defined(OS_WIN)
 #include "base/files/file_path.h"
 #include "base/metrics/histogram_macros.h"
 #include "base/path_service.h"
@@ skipping 122 matching lines @@
 #if defined(OS_WIN)
   // TODO(jwd): Remove this once we're confident most clients no longer have
   // these labels (M57-M58 timeframe).
   // Do the work on a blocking pool thread, as chrome://profiler has shown that
   // it can cause jank if done on the UI thrread.
   content::BrowserThread::GetBlockingPool()->PostDelayedTask(
       FROM_HERE, base::Bind(&ClearGoogleUpdateRegistryLabels),
       base::TimeDelta::FromSeconds(5));
 #endif
 }