Support additional mash shelf alignments.

mash/wm/public/interfaces/shelf_layout.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 mash.wm.mojom;
+
+// These values match ash::ShelfAlignment.
+enum ShelfAlignment { BOTTOM, LEFT, RIGHT, };
+
+// These values match ash::ShelfAutoHideBehavior.
+enum ShelfAutoHideBehavior { ALWAYS, NEVER, HIDDEN, };
+
+// Used by mojo:ash_sysui to push the current shelf state to its layout manager.
+interface ShelfLayout {
+  // TODO: Update preferred sizes with alignment to avoid flicker during layout.
+  SetAlignment(ShelfAlignment alignment);
+  SetAutoHideBehavior(ShelfAutoHideBehavior auto_hide);
+};