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..b23941cdadf67594d09b1b9e988744507ac4cb68 100644
--- a/base/sequence_checker.h
+++ b/base/sequence_checker.h
@@ -29,10 +29,11 @@ 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 or SingleThreadTaskRunners bound
+//   to the same sequence, or,
+// - From a single thread outside of any task.

[gab, 2016/07/26 20:49:48]

I find this last statement confusing, MessageLoop as SingleThreadTaskRunners but
they don't have a "sequence" per se (i.e. no SequenceToken), which category are
they in?

[fdoray, 2016/07/28 18:10:04]

I think we don't plan to support child single-threaded TaskRunners in
base/task_scheduler? If that's true, then this comment could become:

[...] Calls are sequenced when they are issued:
 * From tasks posted to SequencedTaskRunners bound to the same sequence.
(task_scheduler and SequencedWorkerPool can have different SequencedTaskRunners
bound to the same sequence)
 * From tasks posted to the same SingleThreadTaskRunner.
 * From a single thread, outside of any task.

[gab, 2016/07/29 13:31:37]

 On 2016/07/28 18:10:04, fdoray wrote:
I don't think this (..) precision is required.
Otherwise yes sgtm (and we can adjust the comment if we decide to allow
SingleThreadTaskRunners to share a sequence later).

 //
 // Example:
 // class MyClass {