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..264e3b65e34509061f44778af15b882ceb020ca5 100644
--- a/base/sequence_token.cc
+++ b/base/sequence_token.cc
@@ -37,6 +37,10 @@ bool SequenceToken::IsValid() const {
   return token_ != kInvalidSequenceToken;
 }
 
+int SequenceToken::ToInternalValue() const {
+  return token_;
+}
+
 SequenceToken SequenceToken::Create() {
   return SequenceToken(g_sequence_token_generator.GetNext());
 }