mus: Fix shutdown crash in unified desktop mode

Remove and destroy reflectors before closing and deleting hosts.
(roughly: hosts own the surface that reflectors use for painting)

Bug: 806353
Change-Id: Idcbfad0da19f1ccdeabb5d6851e6d55602fdaed1
Reviewed-on: https://chromium-review.googlesource.com/892054Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Michael Wasserman <msw@chromium.org>
Cr-Commit-Position: refs/heads/master@{#533047}

ash/display/mirror_window_controller.cc

@@ -305,15 +305,15 @@ void MirrorWindowController::CloseIfNotNecessary() {
 }
 
 void MirrorWindowController::Close(bool delay_host_deletion) {
-  for (auto& info : mirroring_host_info_map_)
-    CloseAndDeleteHost(info.second, delay_host_deletion);
-
-  mirroring_host_info_map_.clear();
   if (reflector_) {
     aura::Env::GetInstance()->context_factory_private()->RemoveReflector(
         reflector_.get());
     reflector_.reset();
   }
+
+  for (auto& info : mirroring_host_info_map_)
+    CloseAndDeleteHost(info.second, delay_host_deletion);
+  mirroring_host_info_map_.clear();
 }
 
 void MirrorWindowController::OnHostResized(aura::WindowTreeHost* host) {