Expand and split DisplayController mojom.

services/ui/public/interfaces/display/test_display_controller.mojom

+// Copyright 2016 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.
+
+module display.mojom;
+
+// Provides a way to modify the display state at runtime. Will only work when
+// running off device with fake displays.
+interface TestDisplayController {
+
+  // Toggles adding or removing a fake display. If there is only one display
+  // a second display is added. If there is more than one display then the last
+  // display is removed. 
+  ToggleAddRemoveDisplay();
+};