Make WindowTree::SetModal() take the type.

ui/base/mojo/ui_base_types.mojom

Index: ui/base/mojo/ui_base_types.mojom
diff --git a/ui/base/mojo/ui_base_types.mojom b/ui/base/mojo/ui_base_types.mojom
new file mode 100644
index 0000000000000000000000000000000000000000..e2b12c0016454727ebab2605c6a6569146045007
--- /dev/null
+++ b/ui/base/mojo/ui_base_types.mojom
@@ -0,0 +1,21 @@
+// 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.
+
+module ui.mojom;
+
+// Specifies the type of modality applied to a window. Different modal
+// treatments may be handled differently by the window manager.
+enum ModalType {
+  // Window is not modal.
+  NONE,
+
+  // Window is modal to its transient parent.
+  WINDOW,
+
+  // Window is modal to a child of its transient parent.
+  CHILD,
+
+  // Window is modal to all other windows.
+  SYSTEM
+};