Refactors DockedWindowLayoutManager in terms of ash/wm/common

ash/system/user/tray_user.cc

Index: ash/system/user/tray_user.cc
diff --git a/ash/system/user/tray_user.cc b/ash/system/user/tray_user.cc
index 8fe7af40d5cafdc542f6578c35559883ff92b077..9ec90db27ec6b030aca6fe7f0329aa3fbc90afd8 100644
--- a/ash/system/user/tray_user.cc
+++ b/ash/system/user/tray_user.cc
@@ -188,7 +188,7 @@ void TrayUser::UpdateAfterLoginStatusChange(user::LoginStatus status) {
   UpdateLayoutOfItem();
 }
 
-void TrayUser::UpdateAfterShelfAlignmentChange(ShelfAlignment alignment) {
+void TrayUser::UpdateAfterShelfAlignmentChange(wm::ShelfAlignment alignment) {
   // Inactive users won't have a layout.
   if (!layout_view_)
     return;