chromeos: makes it possible for windows to outlive WindowTreeClient

ui/aura/test/aura_test_base.cc

Index: ui/aura/test/aura_test_base.cc
diff --git a/ui/aura/test/aura_test_base.cc b/ui/aura/test/aura_test_base.cc
index 45f49357bc76b9bb6d54dfce21aca66fd7683725..581c74d2e005b47be68d64f1d4818f4dc09b8a4c 100644
--- a/ui/aura/test/aura_test_base.cc
+++ b/ui/aura/test/aura_test_base.cc
@@ -124,6 +124,11 @@ void AuraTestBase::EnableMusWithTestWindowTree() {
   use_mus_ = true;
 }
 
+void AuraTestBase::DeleteWindowTreeClient() {
+  DCHECK(use_mus_);
+  helper_->DeleteWindowTreeClient();
+}
+
 void AuraTestBase::ConfigureBackend(BackendType type) {
   if (type == BackendType::MUS)
     EnableMusWithTestWindowTree();