[reference - do not submit] Always create four pools in TaskSchedulerImpl.

base/task_scheduler/scheduler_single_thread_task_runner_manager.cc

Index: base/task_scheduler/scheduler_single_thread_task_runner_manager.cc
diff --git a/base/task_scheduler/scheduler_single_thread_task_runner_manager.cc b/base/task_scheduler/scheduler_single_thread_task_runner_manager.cc
index 6a2676fb49b9e20bb8f06ea8b52a4839a76a8922..dc9d5379e06c016df4d556361f2a2fe8df316bc3 100644
--- a/base/task_scheduler/scheduler_single_thread_task_runner_manager.cc
+++ b/base/task_scheduler/scheduler_single_thread_task_runner_manager.cc
@@ -207,18 +207,9 @@ class SchedulerSingleThreadTaskRunnerManager::SchedulerSingleThreadTaskRunner
 };
 
 SchedulerSingleThreadTaskRunnerManager::SchedulerSingleThreadTaskRunnerManager(
-    const std::vector<SchedulerWorkerPoolParams>& worker_pool_params_vector,
-    const TaskScheduler::WorkerPoolIndexForTraitsCallback&
-        worker_pool_index_for_traits_callback,
     TaskTracker* task_tracker,
     DelayedTaskManager* delayed_task_manager)
-    : worker_pool_params_vector_(worker_pool_params_vector),
-      worker_pool_index_for_traits_callback_(
-          worker_pool_index_for_traits_callback),
-      task_tracker_(task_tracker),
-      delayed_task_manager_(delayed_task_manager) {
-  DCHECK_GT(worker_pool_params_vector_.size(), 0U);
-  DCHECK(worker_pool_index_for_traits_callback_);
+    : task_tracker_(task_tracker), delayed_task_manager_(delayed_task_manager) {
   DCHECK(task_tracker_);
   DCHECK(delayed_task_manager_);
 }
@@ -236,12 +227,11 @@ SchedulerSingleThreadTaskRunnerManager::
 
 scoped_refptr<SingleThreadTaskRunner>
 SchedulerSingleThreadTaskRunnerManager::CreateSingleThreadTaskRunnerWithTraits(
+    const std::string& name,
+    ThreadPriority priority_hint,
     const TaskTraits& traits) {
-  size_t index = worker_pool_index_for_traits_callback_.Run(traits);
-  DCHECK_LT(index, worker_pool_params_vector_.size());
   return new SchedulerSingleThreadTaskRunner(
-      this, traits,
-      CreateAndRegisterSchedulerWorker(worker_pool_params_vector_[index]));
+      this, traits, CreateAndRegisterSchedulerWorker(name, priority_hint));
 }
 
 void SchedulerSingleThreadTaskRunnerManager::JoinForTesting() {
@@ -264,14 +254,15 @@ void SchedulerSingleThreadTaskRunnerManager::JoinForTesting() {
 
 SchedulerWorker*
 SchedulerSingleThreadTaskRunnerManager::CreateAndRegisterSchedulerWorker(
-    const SchedulerWorkerPoolParams& params) {
+    const std::string& name,
+    ThreadPriority priority_hint) {
   AutoSchedulerLock auto_lock(workers_lock_);
   int id = next_worker_id_++;
-  auto delegate = MakeUnique<SchedulerWorkerDelegate>(base::StringPrintf(
-      "TaskSchedulerSingleThreadWorker%d%s", id, params.name().c_str()));
-  workers_.emplace_back(SchedulerWorker::Create(
-      params.priority_hint(), std::move(delegate), task_tracker_,
-      SchedulerWorker::InitialState::DETACHED));
+  auto delegate = MakeUnique<SchedulerWorkerDelegate>(
+      StringPrintf("TaskScheduler%sSingleThread%d", name.c_str(), id));
+  workers_.emplace_back(
+      SchedulerWorker::Create(priority_hint, std::move(delegate), task_tracker_,
+                              SchedulerWorker::InitialState::DETACHED));
   return workers_.back().get();
 }