TaskScheduler: Add SequenceToken and ScopedSetSequenceTokenForCurrentThread

base/sequence_checker.h

Index: base/sequence_checker.h
diff --git a/base/sequence_checker.h b/base/sequence_checker.h
index ad0182825c24805548f05972c7c04e6813688bc7..6a2ea540f85fdcbba582995b4a40a81f50b5c38e 100644
--- a/base/sequence_checker.h
+++ b/base/sequence_checker.h
@@ -29,10 +29,10 @@ class SequenceCheckerDoNothing {
   void DetachFromSequence() {}
 };
 
-// SequenceChecker is a helper class used to help verify that some
-// methods of a class are called in sequence -- that is, called from
-// the same SequencedTaskRunner. It is a generalization of
-// ThreadChecker; see comments in sequence_checker_impl.h for details.
+// SequenceChecker is a helper class to verify that calls to some methods of a
+// class are sequenced. Calls are sequenced when they are issued from tasks
+// posted to SequencedTaskRunners bound to the same sequence or from a single
+// thread with no associated sequence token.
 //
 // Example:
 // class MyClass {