cros: create TabletPowerButtonControllerTestApi

changes:
TabletPowerButtonControllerTestApi is a nested class, move it to a
separate file.

Bug: none
Test: compiles & ash_unittests
Change-Id: Ic0a31e587e49935ce79cc876f0cdb51a44e78f7d
Reviewed-on: https://chromium-review.googlesource.com/719731
Reviewed-by: James Cook <jamescook@chromium.org>
Commit-Queue: Qiang(Joe) Xu <warx@chromium.org>
Cr-Commit-Position: refs/heads/master@{#508905}

diff --git a/ash/wm/lock_state_controller_test_api.h b/ash/wm/lock_state_controller_test_api.h
index 7bdf0ff0..d0336f9 100644
--- a/ash/wm/lock_state_controller_test_api.h
+++ b/ash/wm/lock_state_controller_test_api.h

@@ -5,15 +5,11 @@
 #ifndef ASH_WM_LOCK_STATE_CONTROLLER_TEST_API_H_
 #define ASH_WM_LOCK_STATE_CONTROLLER_TEST_API_H_
 
-#include <memory>
-
 #include "ash/wm/lock_state_controller.h"
-#include "base/timer/elapsed_timer.h"
-#include "base/timer/timer.h"
 
 namespace ash {
 
-// Helper class used by tests to access internal state.
+// Helper class used by tests to access LockStateController's internal state.
 class LockStateControllerTestApi {
  public:
   explicit LockStateControllerTestApi(LockStateController* controller);