Separate the create and start phases in SchedulerSingleThreadTaskRunnerManager.

base/task_scheduler/task_scheduler_impl.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/task_scheduler_impl.h"
 
 #include <utility>
 
 #include "base/bind.h"
 #include "base/bind_helpers.h"
@@ skipping 222 matching lines @@
 
   // Instantiate DelayedTaskManager. Needs to happen after starting the service
   // thread to get its task_runner().
   delayed_task_manager_ =
       base::MakeUnique<DelayedTaskManager>(service_thread_.task_runner());
 
   single_thread_task_runner_manager_ =
       MakeUnique<SchedulerSingleThreadTaskRunnerManager>(
           worker_pool_params_vector, worker_pool_index_for_traits_callback_,
           task_tracker_.get(), delayed_task_manager_.get());
+  single_thread_task_runner_manager_->Start();

[gab, 2017/04/10 18:04:56]

Just checking: does anything break down if we forget this line :)? i.e. do we
have a safe check against completely breaking integration?

[fdoray, 2017/04/10 19:05:00]

TaskSchedulerImplTest.MultipleTraitsExecutionModePairs verifies that
single-threaded tasks are scheduled, so we can't miss this (I verified that the
test fails when we remove this line).

 
   // Callback invoked by workers to re-enqueue a sequence in the appropriate
   // PriorityQueue.
   const SchedulerWorkerPoolImpl::ReEnqueueSequenceCallback
       re_enqueue_sequence_callback =
           Bind(&TaskSchedulerImpl::ReEnqueueSequenceCallback, Unretained(this));
 
   // Start worker pools.
   for (const auto& worker_pool_params : worker_pool_params_vector) {
     // Passing pointers to objects owned by |this| to
@@ skipping 25 matching lines @@
   // in |sequence|.
   const TaskTraits traits =
       sequence->PeekTaskTraits().WithPriority(sort_key.priority());
 
   GetWorkerPoolForTraits(traits)->ReEnqueueSequence(std::move(sequence),
                                                     sort_key);
 }
 
 }  // namespace internal
 }  // namespace base