Remove unused include in sequenced_worker_pool.h

components/browser_sync/signin_confirmation_helper.h

 // 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.
 
 #ifndef COMPONENTS_BROWSER_SYNC_SIGNIN_CONFIRMATION_HELPER_H_
 #define COMPONENTS_BROWSER_SYNC_SIGNIN_CONFIRMATION_HELPER_H_
 
 #include <stddef.h>
 
 #include "base/macros.h"
 #include "base/memory/ref_counted.h"
 #include "base/task/cancelable_task_tracker.h"
 
+namespace base {
+class SingleThreadTaskRunner;
+}
+
 namespace history {
 class HistoryService;
 class QueryResults;
 }
 
 namespace browser_sync {
 
 // Helper class for sync signin to check some asynchronous conditions. Call
 // either CheckHasHistory or CheckHasTypedUrls or both, and |return_result|
 // will be called with true if either returns true, otherwise false.
@@ skipping 37 matching lines @@
 
   // Callback to pass the result back to the caller.
   const base::Callback<void(bool)> return_result_;
 
   DISALLOW_COPY_AND_ASSIGN(SigninConfirmationHelper);
 };
 
 }  // namespace browser_sync
 
 #endif  // COMPONENTS_BROWSER_SYNC_SIGNIN_CONFIRMATION_HELPER_H_