ash: Do not use MessageLoopForUI when not needed.

ash/display/window_tree_host_manager.cc

Index: ash/display/window_tree_host_manager.cc
diff --git a/ash/display/window_tree_host_manager.cc b/ash/display/window_tree_host_manager.cc
index f99f7a41b1c44ef3c24cdeefc658b9626ac3c977..7575653ba4dd5f9af6d42a6f45656077579b76d2 100644
--- a/ash/display/window_tree_host_manager.cc
+++ b/ash/display/window_tree_host_manager.cc
@@ -33,6 +33,7 @@
 #include "base/stl_util.h"
 #include "base/strings/stringprintf.h"
 #include "base/strings/utf_string_conversions.h"
+#include "base/thread_task_runner_handle.h"
 #include "ui/aura/client/capture_client.h"
 #include "ui/aura/client/focus_client.h"
 #include "ui/aura/client/screen_position_client.h"
@@ -642,7 +643,7 @@ void WindowTreeHostManager::DeleteHost(AshWindowTreeHost* host_to_delete) {
   // Delete most of root window related objects, but don't delete
   // root window itself yet because the stack may be using it.
   controller->Shutdown();
-  base::MessageLoop::current()->DeleteSoon(FROM_HERE, controller);
+  base::ThreadTaskRunnerHandle::Get()->DeleteSoon(FROM_HERE, controller);
 }
 
 void WindowTreeHostManager::OnDisplayRemoved(const gfx::Display& display) {