Update TaskScheduler docs to make it more obvious how a TaskScheduler should be put in place.

base/task_scheduler/post_task.h

Index: base/task_scheduler/post_task.h
diff --git a/base/task_scheduler/post_task.h b/base/task_scheduler/post_task.h
index 8b3e7dfa05b56a7757625f3cec75486ba4401f97..caa100a2605dec1948065f697e64d7b9b15d088a 100644
--- a/base/task_scheduler/post_task.h
+++ b/base/task_scheduler/post_task.h
@@ -23,9 +23,6 @@ namespace base {
 
 // This is the preferred interface to post tasks to the TaskScheduler.
 //
-// TaskScheduler must have been registered for the current process via
-// TaskScheduler::SetInstance() before the functions below are valid.
-//
 // To post a simple one-off task with default traits:
 //     PostTask(FROM_HERE, Bind(...));
 //
@@ -62,6 +59,14 @@ namespace base {
 // registered TaskScheduler (i.e. not on the main thread). Tasks posted through
 // functions below with a delay may be coalesced (i.e. delays may be adjusted to
 // reduce the number of wakeups and hence power consumption).
+//
+// Prerequisite: A TaskScheduler must have been registered for the current
+// process via TaskScheduler::SetInstance() before the functions below are
+// valid. Note: this is typically done in the main of each process and you most

[robliao, 2017/02/23 23:00:33]

Omit "Note:"

Maybe
This is typically done during the initialization phase in each process. If your
code is not running in this phase, you most likely don't have to worry about
this. You will encounter DCHECKs or nullptr dereferences if this is violated.
For tests...

[gab, 2017/02/23 23:39:57]

Done and also documented GetInstance() further to make it clear that
SetInstance() should be done in strictly one place.

+// likely don't have to worry about this if you're not in charge of the main for
+// your process (the failure mode isn't subtle either -- DCHECK or nullptr
+// crash -- so you'll find out quickly if this isn't the case). For tests,
+// prefer base::test::ScopedTaskScheduler.
 
 // Posts |task| to the TaskScheduler. Calling this is equivalent to calling
 // PostTaskWithTraits with plain TaskTraits.