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

base/sequence_checker_impl.h

Index: base/sequence_checker_impl.h
diff --git a/base/sequence_checker_impl.h b/base/sequence_checker_impl.h
index a54c38845114b15a1e78ff097ad116b55d29cb82..74039718f62de96890aeee3c6e6524766523a1c0 100644
--- a/base/sequence_checker_impl.h
+++ b/base/sequence_checker_impl.h
@@ -5,12 +5,13 @@
 #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 {
 
@@ -34,11 +35,22 @@
   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);
 };