Separate the create and start phases in SchedulerSingleThreadTaskRunnerManager.

base/task_scheduler/scheduler_single_thread_task_runner_manager.h

Index: base/task_scheduler/scheduler_single_thread_task_runner_manager.h
diff --git a/base/task_scheduler/scheduler_single_thread_task_runner_manager.h b/base/task_scheduler/scheduler_single_thread_task_runner_manager.h
index 24d1a9a010b35661ee8235666652b16c5d0312a1..c862f19df0b2a46d6abe9d7fc570a8f494ffb5bc 100644
--- a/base/task_scheduler/scheduler_single_thread_task_runner_manager.h
+++ b/base/task_scheduler/scheduler_single_thread_task_runner_manager.h
@@ -35,6 +35,12 @@ class SchedulerWorkerDelegate;
 
 }  // namespace
 
+// Manages a pool of threads which are each associated with one
+// SingleThreadTaskRunner.
+//
+// No threads are created (and hence no tasks can run) before Start() is called.
+//
+// This class is thread-safe.
 class BASE_EXPORT SchedulerSingleThreadTaskRunnerManager final {
  public:
   SchedulerSingleThreadTaskRunnerManager(
@@ -45,6 +51,10 @@ class BASE_EXPORT SchedulerSingleThreadTaskRunnerManager final {
       DelayedTaskManager* delayed_task_manager);
   ~SchedulerSingleThreadTaskRunnerManager();
 
+  // Starts threads for existing SingleThreadTaskRunners and allows threads to
+  // be started when SingleThreadTaskRunners are created in the future.
+  void Start();
+
   scoped_refptr<SingleThreadTaskRunner> CreateSingleThreadTaskRunnerWithTraits(
       const TaskTraits& traits);
 
@@ -79,11 +89,14 @@ class BASE_EXPORT SchedulerSingleThreadTaskRunnerManager final {
   TaskTracker* const task_tracker_;
   DelayedTaskManager* const delayed_task_manager_;
 
-  // Synchronizes access to |workers_| and |worker_id_|.
-  SchedulerLock workers_lock_;
+  // Synchronizes access to |workers_|, |next_worker_id_| and |started_|.
+  SchedulerLock lock_;
   std::vector<scoped_refptr<SchedulerWorker>> workers_;
   int next_worker_id_ = 0;
 
+  // Set to true when Start() is called.
+  bool started_ = false;
+
 #if DCHECK_IS_ON()
   subtle::Atomic32 workers_unregistered_during_join_ = 0;
 #endif