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

base/sequence_token.cc

Index: base/sequence_token.cc
diff --git a/base/sequence_token.cc b/base/sequence_token.cc
index 1178181ddcafbad2583d9acb7c43a48990025a53..a3e8fd1bc49ea530e529a926599706d875dc3783 100644
--- a/base/sequence_token.cc
+++ b/base/sequence_token.cc
@@ -7,6 +7,7 @@
 #include "base/atomic_sequence_num.h"
 #include "base/lazy_instance.h"
 #include "base/logging.h"
+#include "base/strings/string_number_conversions.h"
 #include "base/threading/thread_local.h"
 
 namespace base {
@@ -37,6 +38,10 @@ bool SequenceToken::IsValid() const {
   return token_ != kInvalidSequenceToken;
 }
 
+std::string SequenceToken::ToString() const {
+  return IntToString(token_);
+}
+
 SequenceToken SequenceToken::Create() {
   return SequenceToken(g_sequence_token_generator.GetNext());
 }