[MessageLoop]: Replace MessageLoopCurrent::task_runner() in /ash/app_list.

MessageLoopCurrent::task_runner() is depecated.
This CL replaces it with with ThreadTaskRunnerHandle::Get().
A small step towards deleting MessageLoop.

This CL was generated with git cl split.
Refer to the top-level CL if necessary :
https://chromium-review.googlesource.com/c/chromium/src/+/1631693

Please CQ if LGTY!

This CL was uploaded by git cl split.

R=newcomer@chromium.org

Bug: 616447
Change-Id: I8cfb4b77da06edc6ed251bcad917f6e4369fc7f1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1636063
Auto-Submit: Etienne Pierre-Doray <etiennep@chromium.org>
Reviewed-by: Alex Newcomer <newcomer@chromium.org>
Commit-Queue: Alex Newcomer <newcomer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#664840}

ash/app_list/demo/app_list_demo_views.cc

@@ -11,9 +11,9 @@
 #include "ash/app_list/views/app_list_view.h"
 #include "base/bind.h"
 #include "base/macros.h"
-#include "base/message_loop/message_loop_current.h"
 #include "base/run_loop.h"
 #include "base/single_thread_task_runner.h"
+#include "base/threading/thread_task_runner_handle.h"
 #include "build/build_config.h"
 #include "ui/base/resource/resource_bundle.h"
 #include "ui/resources/grit/ui_resources.h"
@@ -73,8 +73,7 @@ void DemoAppListViewDelegate::DismissAppList() {
 }
 
 void DemoAppListViewDelegate::ViewClosing() {
-  base::MessageLoopCurrent message_loop = base::MessageLoopCurrentForUI::Get();
-  message_loop->task_runner()->DeleteSoon(FROM_HERE, this);
+  base::ThreadTaskRunnerHandle::Get()->DeleteSoon(FROM_HERE, this);
   base::RunLoop::QuitCurrentWhenIdleDeprecated();
 }