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 <string>
+
 #include "base/base_export.h"
 #include "base/macros.h"
 
 namespace base {
 
 // A token that identifies a series of sequenced tasks (i.e. tasks that run one
 // at a time in posting order).
 class BASE_EXPORT SequenceToken {
  public:
   // Instantiates an invalid SequenceToken.
   SequenceToken() = default;
 
   // Explicitly allow copy.
   SequenceToken(const SequenceToken& other) = default;
   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 a string with a number uniquely representing this SequenceToken.

[fdoray, 2016/10/05 12:39:40]

Nobody should use this for logic. Maybe add:

// Should only be used for tracing, profiling and debugging.

Similar to
https://cs.chromium.org/chromium/src/base/threading/sequenced_worker_pool.h?l...

[gab, 2016/10/05 20:12:43]

Done.

+  std::string ToString() 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_