Add more tests to test OverviewButtonTray.

ash/system/overview/test/overview_button_tray_test_api.cc

+// Copyright 2017 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "ash/system/overview/test/overview_button_tray_test_api.h"
+
+#include "ash/system/overview/overview_button_tray.h"
+#include "ui/compositor/layer_animator.h"
+
+namespace ash {
+namespace test {
+
+OverviewButtonTrayTestApi::OverviewButtonTrayTestApi(OverviewButtonTray* tray)
+    : tray_(tray) {}
+
+void OverviewButtonTrayTestApi::AbortAllAnimations() {
+  tray_->layer()->GetAnimator()->AbortAllAnimations();
+}
+
+}  // namespace test
+}  // namespace ash