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

base/sequence_token.h

Index: base/sequence_token.h
diff --git a/base/sequence_token.h b/base/sequence_token.h
index 80340c0f11f07ed359463c1bd941fa2ba9bd04b0..115a883384b13aede8082b31db1abc23460b7073 100644
--- a/base/sequence_token.h
+++ b/base/sequence_token.h
@@ -5,6 +5,8 @@
 #ifndef BASE_SEQUENCE_TOKEN_H_
 #define BASE_SEQUENCE_TOKEN_H_
 
+#include <string>
+
 #include "base/base_export.h"
 #include "base/macros.h"
 
@@ -29,6 +31,9 @@ class BASE_EXPORT SequenceToken {
   // 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();