Fix destruction order in mus_demo

services/ui/demo/mus_demo.cc

Index: services/ui/demo/mus_demo.cc
diff --git a/services/ui/demo/mus_demo.cc b/services/ui/demo/mus_demo.cc
index 7afba5693af4cae100e7e13c0f09af916494b989..0efcf0068d68e830a5b12cd917a1d22c381f8f64 100644
--- a/services/ui/demo/mus_demo.cc
+++ b/services/ui/demo/mus_demo.cc
@@ -24,6 +24,14 @@ MusDemo::MusDemo() {}
 
 MusDemo::~MusDemo() {
   display::Screen::SetScreenInstance(nullptr);
+  // Destruction order is important here:
+  // 1) Windows must be destroyed before WindowTreeClient's destructor is
+  // called.
+  // 2) WindowTreeClient must be destroyed before Env and WMState.
+  window_tree_data_list_.clear();
+  window_tree_client_.reset();
+  env_.reset();
+  wm_state_.reset();
 }
 
 void MusDemo::AddPrimaryDisplay(const display::Display& display) {