ash: Do not use MessageLoopForUI when not needed.

ash/accelerators/key_hold_detector.cc

Index: ash/accelerators/key_hold_detector.cc
diff --git a/ash/accelerators/key_hold_detector.cc b/ash/accelerators/key_hold_detector.cc
index 0c8d1ed337798620b385fbdd8973973f8854d774..91391502f99be473e8eb61f44d97935a3799d7ce 100644
--- a/ash/accelerators/key_hold_detector.cc
+++ b/ash/accelerators/key_hold_detector.cc
@@ -7,7 +7,7 @@
 #include <utility>
 
 #include "ash/shell.h"
-#include "base/message_loop/message_loop.h"
+#include "base/thread_task_runner_handle.h"
 #include "ui/aura/window_tracker.h"
 #include "ui/aura/window_tree_host.h"
 #include "ui/events/event_dispatcher.h"
@@ -37,7 +37,7 @@ void PostPressedEvent(ui::KeyEvent* event) {
   scoped_ptr<aura::WindowTracker> tracker(new aura::WindowTracker);
   tracker->Add(static_cast<aura::Window*>(event->target()));
 
-  base::MessageLoopForUI::current()->PostTask(

[oshima, 2016/01/27 22:23:53]

My understanding was that we used to use MessageLoopForUI::current() as a safe
guard to make sure that UI code runs on single UI thread.

I understand that it does not really have to be "UI thread" in new world, but
still should be single thread. Is there a way to make sure they runs on the same
single thread?

[sadrul, 2016/01/27 23:44:02]

ThreadTaskRunnerHandle::Get() returns the task-runner for the current thread,
and it's a SingleThreadTaskRunner instance. So we are guaranteed to run the task
(i.e. DispatchPressedEvent) in the same thread as this code (i.e.
PostPressedEvent).

Is that what you mean? We can also use base::ThreadChecker to verify that parts
of the code are running in the thread we expect, but it's not clear where we
would use that in this case.

+  base::ThreadTaskRunnerHandle::Get()->PostTask(
       FROM_HERE,
       base::Bind(&DispatchPressedEvent, pressed_event, base::Passed(&tracker)));
 }