Add ExecutionMode/SequenceToken and TaskPriority as arguments of TaskScheduler tasks in tracing

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.
+  std::string ToString() const;
+
   // Returns a valid SequenceToken which isn't equal to any previously returned
   // SequenceToken.
   static SequenceToken Create();