Adds a SHUTDOWN_TASK_QUEUE and a PreShutdown api to the scheduler.

components/scheduler/child/webthread_impl_for_worker_scheduler.cc

Index: components/scheduler/child/webthread_impl_for_worker_scheduler.cc
diff --git a/components/scheduler/child/webthread_impl_for_worker_scheduler.cc b/components/scheduler/child/webthread_impl_for_worker_scheduler.cc
index c293faae69d38d46e577a76fc34758616a669aea..aec0b2eac24fc9dc3ca8f01f2ab3a446b0e9a77c 100644
--- a/components/scheduler/child/webthread_impl_for_worker_scheduler.cc
+++ b/components/scheduler/child/webthread_impl_for_worker_scheduler.cc
@@ -27,9 +27,10 @@ WebThreadImplForWorkerScheduler::WebThreadImplForWorkerScheduler(
 
 WebThreadImplForWorkerScheduler::~WebThreadImplForWorkerScheduler() {
   base::WaitableEvent completion(false, false);
-  // We need to post the shutdown task on the scheduler's task queue or tasks
-  // posted on the worker scheduler may not get run when the thread is deleted.
-  TaskRunner()->PostTask(
+  // We need to post the shutdown task on the scheduler's shutdown task queue or
+  // other (shutdown) tasks posted on the worker scheduler may not get run when
+  // the thread is deleted.
+  worker_scheduler_->ShutdownTaskRunner()->PostTask(
       FROM_HERE, base::Bind(&WebThreadImplForWorkerScheduler::ShutDownOnThread,
                             base::Unretained(this), &completion));
   completion.Wait();
@@ -46,7 +47,8 @@ void WebThreadImplForWorkerScheduler::InitOnThread(
   web_scheduler_.reset(new WebSchedulerImpl(
       worker_scheduler_.get(), worker_scheduler_->IdleTaskRunner(),
       worker_scheduler_->DefaultTaskRunner(),
-      worker_scheduler_->DefaultTaskRunner()));
+      worker_scheduler_->DefaultTaskRunner(),
+      worker_scheduler_->ShutdownTaskRunner()));
   completion->Signal();
 }