aura-mus: Implement DesktopWindowTreeHostMus::StackAbove().

services/ui/ws/test_utils.cc

Index: services/ui/ws/test_utils.cc
diff --git a/services/ui/ws/test_utils.cc b/services/ui/ws/test_utils.cc
index 1506bdcb8a974b461c9bb8ef6876088848bc65cd..5d87a75389fd6d894288ef6ea05d9b7b8ccc88de 100644
--- a/services/ui/ws/test_utils.cc
+++ b/services/ui/ws/test_utils.cc
@@ -241,6 +241,10 @@ void TestWindowManager::WmCancelMoveLoop(uint32_t window_id) {}
 
 void TestWindowManager::WmDeactivateWindow(uint32_t window_id) {}
 
+void TestWindowManager::WmStackAbove(uint32_t change_id,
+                                     uint32_t above_id,
+                                     uint32_t below_id) {}
+
 void TestWindowManager::WmStackAtTop(uint32_t change_id, uint32_t window_id) {}
 
 void TestWindowManager::OnAccelerator(uint32_t ack_id,