ash: Do not use MessageLoopForUI when not needed.

ash/system/tray/system_tray_bubble.cc

Index: ash/system/tray/system_tray_bubble.cc
diff --git a/ash/system/tray/system_tray_bubble.cc b/ash/system/tray/system_tray_bubble.cc
index 6f95ba29edc617b1f971a64d654094dc171ed84d..eab4b1cf98a0d3fba611c8f35d8b1b3713cb410d 100644
--- a/ash/system/tray/system_tray_bubble.cc
+++ b/ash/system/tray/system_tray_bubble.cc
@@ -11,7 +11,7 @@
 #include "ash/system/tray/tray_bubble_wrapper.h"
 #include "ash/system/tray/tray_constants.h"
 #include "ash/system/tray/tray_popup_item_container.h"
-#include "base/message_loop/message_loop.h"
+#include "base/thread_task_runner_handle.h"
 #include "ui/aura/window.h"
 #include "ui/compositor/layer.h"
 #include "ui/compositor/layer_animation_observer.h"
@@ -49,7 +49,7 @@ class AnimationObserverDeleteLayer : public ui::ImplicitAnimationObserver {
   ~AnimationObserverDeleteLayer() override {}
 
   void OnImplicitAnimationsCompleted() override {
-    base::MessageLoopForUI::current()->DeleteSoon(FROM_HERE, this);
+    base::ThreadTaskRunnerHandle::Get()->DeleteSoon(FROM_HERE, this);
   }
 
  private: