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diff --git a/base/task_scheduler/sequence.h b/base/task_scheduler/sequence.h
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+// Copyright 2016 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_TASK_SCHEDULER_SEQUENCE_H_
+#define BASE_TASK_SCHEDULER_SEQUENCE_H_
+
+#include <stddef.h>
+
+#include <queue>
+
+#include "base/base_export.h"
+#include "base/macros.h"
+#include "base/memory/ref_counted.h"
+#include "base/memory/scoped_ptr.h"
+#include "base/task_scheduler/scheduler_lock.h"
+#include "base/task_scheduler/sequence_sort_key.h"
+#include "base/task_scheduler/task.h"
+#include "base/task_scheduler/task_traits.h"
+
+namespace base {
+namespace internal {
+
+// A sequence holds tasks that must be executed in posting order.
+// This class is thread-safe.
+class BASE_EXPORT Sequence : public RefCountedThreadSafe<Sequence> {
+ public:
+  Sequence();
+
+  // Adds |task| at the end of the sequence's queue. Returns true if the
+  // sequence was empty before this operation.
+  bool PushTask(scoped_ptr<Task> task);
+
+  // Returns the task in front of the sequence's queue, if any.
+  const Task* PeekTask() const;
+
+  // Removes the task in front of the sequence's queue. Returns true if the
+  // sequence is empty after this operation. Cannot be called on an empty
+  // sequence.
+  bool PopTask();
+
+  // Returns a SequenceSortKey representing the priority of the sequence. Cannot
+  // be called on an empty sequence.
+  SequenceSortKey GetSortKey() const;
+
+ private:
+  friend class RefCountedThreadSafe<Sequence>;
+  ~Sequence();
+
+  // Synchronizes access to all members.
+  mutable SchedulerLock lock_;
+
+  // Queue of tasks to execute.
+  std::queue<scoped_ptr<Task>> queue_;
+
+  // Number of tasks contained in the sequence for each priority.
+  size_t num_tasks_per_priority_[static_cast<int>(TaskPriority::HIGHEST) + 1] =
+      {};
+
+  DISALLOW_COPY_AND_ASSIGN(Sequence);
+};
+
+}  // namespace internal
+}  // namespace base
+
+#endif  // BASE_TASK_SCHEDULER_SEQUENCE_H_