Converts MruWindowTracker to work with common types

ash/wm/window_util.cc

Index: ash/wm/window_util.cc
diff --git a/ash/wm/window_util.cc b/ash/wm/window_util.cc
index ec27d21ada41a17c1efeb01fe453990059ef4c4f..09eadb5d6d65ec3730c18fdba93c928469665263 100644
--- a/ash/wm/window_util.cc
+++ b/ash/wm/window_util.cc
@@ -58,10 +58,6 @@ bool CanActivateWindow(aura::Window* window) {
   return ::wm::CanActivateWindow(window);
 }
 
-bool IsWindowMinimized(aura::Window* window) {
-  return ash::wm::GetWindowState(window)->IsMinimized();
-}
-
 bool IsWindowUserPositionable(aura::Window* window) {
   return GetWindowState(window)->IsUserPositionable();
 }