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..6e7d191ae8a83f4ab383291757522bc4bb84192d 100644
--- a/base/sequence_token.h
+++ b/base/sequence_token.h
@@ -29,6 +29,10 @@ class BASE_EXPORT SequenceToken {
   // 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();