Revert of Redirect the MessageLoop's task runner to the renderer scheduler

components/scheduler/child/scheduler_helper.cc

Index: components/scheduler/child/scheduler_helper.cc
diff --git a/components/scheduler/child/scheduler_helper.cc b/components/scheduler/child/scheduler_helper.cc
index fb1185d6b01a3ebb58f4ddd3f63b05b42b9a925a..0e13c7635264e98231717d0933d1d0b0d57e22e2 100644
--- a/components/scheduler/child/scheduler_helper.cc
+++ b/components/scheduler/child/scheduler_helper.cc
@@ -7,18 +7,17 @@
 #include "base/time/default_tick_clock.h"
 #include "base/trace_event/trace_event.h"
 #include "base/trace_event/trace_event_argument.h"
-#include "components/scheduler/child/scheduler_task_runner_delegate.h"
+#include "components/scheduler/child/nestable_single_thread_task_runner.h"
 
 namespace scheduler {
 
 SchedulerHelper::SchedulerHelper(
-    scoped_refptr<SchedulerTaskRunnerDelegate> main_task_runner,
+    scoped_refptr<NestableSingleThreadTaskRunner> main_task_runner,
     const char* tracing_category,
     const char* disabled_by_default_tracing_category,
     const char* disabled_by_default_verbose_tracing_category,
     size_t total_task_queue_count)
-    : main_task_runner_(main_task_runner),
-      task_queue_selector_(new PrioritizingTaskQueueSelector()),
+    : task_queue_selector_(new PrioritizingTaskQueueSelector()),
       task_queue_manager_(
           new TaskQueueManager(total_task_queue_count,
                                main_task_runner,
@@ -65,19 +64,14 @@
 
   // TODO(skyostil): Increase this to 4 (crbug.com/444764).
   task_queue_manager_->SetWorkBatchSize(1);
-
-  main_task_runner_->SetDefaultTaskRunner(
-      task_queue_manager_->TaskRunnerForQueue(QueueId::DEFAULT_TASK_QUEUE));
 }
 
 SchedulerHelper::~SchedulerHelper() {
-  Shutdown();
 }
 
 void SchedulerHelper::Shutdown() {
   CheckOnValidThread();
   task_queue_manager_.reset();
-  main_task_runner_->RestoreDefaultTaskRunner();
 }
 
 scoped_refptr<base::SingleThreadTaskRunner>