Revert of Remove header dependencies from sequence_checker.h to sequenced_worker_pool.h

base/sequence_checker_impl.h

 // 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.
 
 #ifndef BASE_SEQUENCE_CHECKER_IMPL_H_
 #define BASE_SEQUENCE_CHECKER_IMPL_H_
 
-#include <memory>
-
 #include "base/base_export.h"
 #include "base/compiler_specific.h"
 #include "base/macros.h"
+#include "base/sequence_token.h"
 #include "base/synchronization/lock.h"
+#include "base/threading/sequenced_worker_pool.h"
+#include "base/threading/thread_checker_impl.h"
 
 namespace base {
 
 // Real implementation of SequenceChecker for use in debug mode or for temporary
 // use in release mode (e.g. to CHECK on a threading issue seen only in the
 // wild).
 //
 // Note: You should almost always use the SequenceChecker class to get the right
 // version for your build configuration.
 class BASE_EXPORT SequenceCheckerImpl {
  public:
   SequenceCheckerImpl();
   ~SequenceCheckerImpl();
 
   // Returns true if called in sequence with previous calls to this method and
   // the constructor.
   bool CalledOnValidSequence() const WARN_UNUSED_RESULT;
 
   // Unbinds the checker from the currently associated sequence. The checker
   // will be re-bound on the next call to CalledOnValidSequence().
   void DetachFromSequence();
 
  private:
-  class Core;
+  void EnsureSequenceTokenAssigned() const;
 
   // Guards all variables below.
   mutable Lock lock_;
-  mutable std::unique_ptr<Core> core_;
+
+  // True when the SequenceChecker is bound to a sequence or a thread.
+  mutable bool is_assigned_ = false;
+
+  mutable SequenceToken sequence_token_;
+
+  // TODO(gab): Remove this when SequencedWorkerPool is deprecated in favor of
+  // TaskScheduler. crbug.com/622400
+  mutable SequencedWorkerPool::SequenceToken sequenced_worker_pool_token_;
+
+  // Used when |sequenced_worker_pool_token_| and |sequence_token_| are invalid.
+  ThreadCheckerImpl thread_checker_;
 
   DISALLOW_COPY_AND_ASSIGN(SequenceCheckerImpl);
 };
 
 }  // namespace base
 
 #endif  // BASE_SEQUENCE_CHECKER_IMPL_H_