Add a task_scheduler tracing category which will record an extra event per task.

base/sequence_token.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_SEQUENCE_TOKEN_H_
 #define BASE_SEQUENCE_TOKEN_H_
 
 #include "base/base_export.h"
 #include "base/macros.h"
 
@@ skipping 11 matching lines @@
   SequenceToken& operator=(const SequenceToken& other) = default;
 
   // An invalid SequenceToken is not equal to any other SequenceToken, including
   // other invalid SequenceTokens.
   bool operator==(const SequenceToken& other) const;
   bool operator!=(const SequenceToken& other) const;
 
   // Returns true if this is a valid SequenceToken.
   bool IsValid() const;
 
+  // Returns the integer uniquely representing this SequenceToken. This method
+  // should only be used for tracing and debugging.
+  int ToInternalValue() const;
+
   // Returns a valid SequenceToken which isn't equal to any previously returned
   // SequenceToken.
   static SequenceToken Create();
 
   // Returns the SequenceToken associated with the task running on the current
   // thread, as determined by the active ScopedSetSequenceTokenForCurrentThread
   // if any.
   static SequenceToken GetForCurrentThread();
 
  private:
@@ skipping 60 matching lines @@
  private:
   const SequenceToken sequence_token_;
   const TaskToken task_token_;
 
   DISALLOW_COPY_AND_ASSIGN(ScopedSetSequenceTokenForCurrentThread);
 };
 
 }  // namespace base
 
 #endif  // BASE_SEQUENCE_TOKEN_H_