Separate the create and start phases in SchedulerWorkerPoolImpl.

base/task_scheduler/priority_queue.h

Index: base/task_scheduler/priority_queue.h
diff --git a/base/task_scheduler/priority_queue.h b/base/task_scheduler/priority_queue.h
index 2844ef3fa5442a9b7ba93b15c72626f46e187a83..d882364b2802914b98d69900b20871652c867ff0 100644
--- a/base/task_scheduler/priority_queue.h
+++ b/base/task_scheduler/priority_queue.h
@@ -55,6 +55,9 @@ class BASE_EXPORT PriorityQueue {
     // Returns true if the PriorityQueue is empty.
     bool IsEmpty() const;
 
+    // Returns the number of Sequences in the PriorityQueue.
+    size_t Size() const;
+
    private:
     friend class PriorityQueue;