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

base/task_scheduler/post_task.cc

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "base/task_scheduler/post_task.h"
 
 #include <utility>
 
+#include "base/logging.h"
 #include "base/task_scheduler/task_scheduler.h"
 #include "base/threading/post_task_and_reply_impl.h"
 
 namespace base {
 
 namespace {
 
 class PostTaskAndReplyTaskRunner : public internal::PostTaskAndReplyImpl {
  public:
   explicit PostTaskAndReplyTaskRunner(const TaskTraits& traits)
@@ skipping 32 matching lines @@
 void PostTaskWithTraits(const tracked_objects::Location& from_here,
                         const TaskTraits& traits,
                         const Closure& task) {
   PostDelayedTaskWithTraits(from_here, traits, task, TimeDelta());
 }
 
 void PostDelayedTaskWithTraits(const tracked_objects::Location& from_here,
                                const TaskTraits& traits,
                                const Closure& task,
                                TimeDelta delay) {
+  DCHECK(TaskScheduler::GetInstance())
+      << "Ref. Prerequisite section of post_task.h";
   TaskScheduler::GetInstance()->PostDelayedTaskWithTraits(from_here, traits,
                                                           task, delay);
 }
 
 void PostTaskWithTraitsAndReply(const tracked_objects::Location& from_here,
                                 const TaskTraits& traits,
                                 Closure task,
                                 Closure reply) {
   PostTaskAndReplyTaskRunner(traits).PostTaskAndReply(
       from_here, std::move(task), std::move(reply));
 }
 
 scoped_refptr<TaskRunner> CreateTaskRunnerWithTraits(const TaskTraits& traits) {
+  DCHECK(TaskScheduler::GetInstance())
+      << "Ref. Prerequisite section of post_task.h";
   return TaskScheduler::GetInstance()->CreateTaskRunnerWithTraits(traits);
 }
 
 scoped_refptr<SequencedTaskRunner> CreateSequencedTaskRunnerWithTraits(
     const TaskTraits& traits) {
+  DCHECK(TaskScheduler::GetInstance())
+      << "Ref. Prerequisite section of post_task.h";
   return TaskScheduler::GetInstance()->CreateSequencedTaskRunnerWithTraits(
       traits);
 }
 
 scoped_refptr<SingleThreadTaskRunner> CreateSingleThreadTaskRunnerWithTraits(
     const TaskTraits& traits) {
+  DCHECK(TaskScheduler::GetInstance())
+      << "Ref. Prerequisite section of post_task.h";
   return TaskScheduler::GetInstance()->CreateSingleThreadTaskRunnerWithTraits(
       traits);
 }
 
 }  // namespace base