Separate the create and start phases in SchedulerSingleThreadTaskRunnerManager.

base/task_scheduler/scheduler_worker_pool_impl.h

Index: base/task_scheduler/scheduler_worker_pool_impl.h
diff --git a/base/task_scheduler/scheduler_worker_pool_impl.h b/base/task_scheduler/scheduler_worker_pool_impl.h
index f9b04a1d2fd3076e733f9fc9efada51c063fbc09..7b285a54d38a84d6380f97bc19e4945f414cae9d 100644
--- a/base/task_scheduler/scheduler_worker_pool_impl.h
+++ b/base/task_scheduler/scheduler_worker_pool_impl.h
@@ -147,12 +147,12 @@ class BASE_EXPORT SchedulerWorkerPoolImpl : public SchedulerWorkerPool {
 
   // All workers owned by this worker pool. Initialized by Start() within the
   // scope of |idle_workers_stack_lock_|. Never modified afterwards (i.e. can be
-  // read without synchronization once |workers_created_.IsSignaled()|).
+  // read without synchronization once |workers_created_.IsSet()|).
   std::vector<scoped_refptr<SchedulerWorker>> workers_;
 
   // Suggested reclaim time for workers. Initialized by Start(). Never modified
   // afterwards (i.e. can be read without synchronization once
-  // |workers_created_.IsSignaled()|).
+  // |workers_created_.IsSet()|).
   TimeDelta suggested_reclaim_time_;
 
   // Synchronizes access to |idle_workers_stack_|,
@@ -172,7 +172,9 @@ class BASE_EXPORT SchedulerWorkerPoolImpl : public SchedulerWorkerPool {
   // Signaled when all workers become idle.
   std::unique_ptr<ConditionVariable> idle_workers_stack_cv_for_testing_;
 
-  // Number of wake ups that occurred before Start().
+  // Number of wake ups that occurred before Start(). Never modified after
+  // Start() (i.e. can be read without synchronization once
+  // |workers_created_.IsSet()|).
   int num_wake_ups_before_start_ = 0;
 
   // Signaled once JoinForTesting() has returned.
@@ -183,8 +185,8 @@ class BASE_EXPORT SchedulerWorkerPoolImpl : public SchedulerWorkerPool {
   AtomicFlag worker_detachment_disallowed_;
 
 #if DCHECK_IS_ON()
-  // Signaled when all workers have been created.
-  mutable WaitableEvent workers_created_;
+  // Set when all workers have been created.
+  AtomicFlag workers_created_;
 #endif
 
   // TaskScheduler.DetachDuration.[worker pool name] histogram. Intentionally