TaskScheduler: Add SequenceToken and ScopedSetSequenceTokenForCurrentThread

base/threading/thread_checker.h

Index: base/threading/thread_checker.h
diff --git a/base/threading/thread_checker.h b/base/threading/thread_checker.h
index 1d970f093ed74b4d8f71c8d0c34006a10f171f3f..87c26ec052dda948d63f376d3c65df476c6fee79 100644
--- a/base/threading/thread_checker.h
+++ b/base/threading/thread_checker.h
@@ -63,6 +63,15 @@ class ThreadCheckerDoNothing {
 //   ThreadChecker thread_checker_;
 // }
 //
+// Note that, when enabled, CalledOnValidThread() returns false when called from
+// tasks posted to SingleThreadTaskRunners bound to different sequences, even if
+// the tasks happen to run on the same thread (e.g. two independent TaskRunners
+// with ExecutionMode::SINGLE_THREADED on the TaskScheduler that happen to share
+// a thread). Also, CalledOnValidThread() returns false when called from a non-
+// single-threaded task which is associated with a SequenceToken (e.g. a task
+// posted with ExecutionMode::PARALLEL or ExecutionMode::SEQUENCED to the

[gab, 2016/07/21 21:14:24]

PARALLEL won't have a SequenceToken -- keeping the example about SEQUENCED is
fine IMO

[fdoray, 2016/07/25 13:24:28]

keeping the example about SEQUENCED to avoid confusion sgtm. But PARALLEL tasks
are in a single-task sequence and hence have a SequenceToken.

[gab, 2016/07/25 14:09:31]

Ah right, but IMO that's an implementation detail of the scheduler (because the
queued type internally happens to be sequences). Don't think we should make API
guarantees about that though.

+// TaskScheduler).
+//
 // In Release mode, CalledOnValidThread will always return true.
 #if ENABLE_THREAD_CHECKER
 class ThreadChecker : public ThreadCheckerImpl {