TaskScheduler: Set the current SequenceToken when a Task runs.

base/task_scheduler/sequence.h

 // 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 <memory>
 #include <queue>
 
 #include "base/base_export.h"
 #include "base/macros.h"
 #include "base/memory/ref_counted.h"
+#include "base/sequence_token.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.
 //
@@ skipping 23 matching lines @@
 
   // 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;
 
+  // Returns a token that uniquely identifies this Sequence.
+  const SequenceToken& token() const { return token_; }
+
  private:
   friend class RefCountedThreadSafe<Sequence>;
   ~Sequence();
 
+  const SequenceToken token_ = SequenceToken::Create();
+
   // Synchronizes access to all members.
   mutable SchedulerLock lock_;
 
   // Queue of tasks to execute.
   std::queue<std::unique_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_