Refactor most recently used window tracking into a separate class.

ash/focus_cycler.cc

Index: ash/focus_cycler.cc
diff --git a/ash/focus_cycler.cc b/ash/focus_cycler.cc
index 81611dc8020d41b64d403b1faa7b8324b0a8ef43..d15b689a17c8cb12b3f7d22cce330f19e2cfb03a 100644
--- a/ash/focus_cycler.cc
+++ b/ash/focus_cycler.cc
@@ -5,6 +5,7 @@
 #include "ash/focus_cycler.h"
 
 #include "ash/shell.h"
+#include "ash/wm/mru_window_tracker.h"
 #include "ash/wm/window_cycle_controller.h"
 #include "ash/wm/window_util.h"
 #include "ui/aura/client/activation_client.h"
@@ -18,7 +19,7 @@ namespace ash {
 namespace {
 
 bool HasFocusableWindow() {
-  return !WindowCycleController::BuildWindowList(NULL, false).empty();
+  return !MruWindowTracker::BuildWindowList(NULL, false).empty();
 }
 
 }  // namespace