TaskScheduler: Make the worker pools of TaskSchedulerImpl configurable

base/task_scheduler/task_scheduler.h

Index: base/task_scheduler/task_scheduler.h
diff --git a/base/task_scheduler/task_scheduler.h b/base/task_scheduler/task_scheduler.h
index b8da64b891a952a4cc629ccae962c32c765c9645..e344e6f9f43fbfd2c9398fffec31200384d9746c 100644
--- a/base/task_scheduler/task_scheduler.h
+++ b/base/task_scheduler/task_scheduler.h
@@ -5,7 +5,9 @@
 #ifndef BASE_TASK_SCHEDULER_TASK_SCHEDULER_H_
 #define BASE_TASK_SCHEDULER_TASK_SCHEDULER_H_
 
+#include <map>
 #include <memory>
+#include <string>
 
 #include "base/base_export.h"
 #include "base/callback_forward.h"
@@ -60,8 +62,10 @@ class BASE_EXPORT TaskScheduler {
   // API for this process.
   static void SetInstance(std::unique_ptr<TaskScheduler> task_scheduler);
 
-  // Initializes the default task scheduler for this process.
-  static void InitializeDefaultTaskScheduler();
+  // Initializes the default task scheduler for this process. |variation_params|
+  // is used to configure the scheduler (can be empty).
+  static void InitializeDefaultTaskScheduler(
+      const std::map<std::string, std::string>& variation_params);
 
   // Retrieve the TaskScheduler set via SetInstance() or
   // InitializeDefaultTaskScheduler(). This should be used very rarely; most