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;
+
+enum ShelfAlignment {
+  SHELF_ALIGNMENT_BOTTOM,
+  SHELF_ALIGNMENT_LEFT,
+  SHELF_ALIGNMENT_RIGHT,
+  // TODO(msw): Why even bother including? SHELF_ALIGNMENT_TOP,

[James Cook, 2016/03/29 00:28:30]

Don't bother, ALIGNMENT_TOP has never been supported. It was taking too long to
build and didn't work well with the tab strip right below it, so we scrapped it
in the middle of Ash development.

[msw, 2016/04/11 22:20:27]

Done.

+};
+
+enum ShelfAutoHideBehavior {
+  SHELF_AUTO_HIDE_BEHAVIOR_ALWAYS,
+  SHELF_AUTO_HIDE_BEHAVIOR_NEVER,
+  SHELF_AUTO_HIDE_ALWAYS_HIDDEN,
+};
+
+// Used to push desired shelf state to the shelf container's layout manager.

[James Cook, 2016/03/29 00:28:29]

nit: Is this really "desired"? It seems like sysui is just telling the WM --
hey, this is the current shelf layout, deal with it.  Also, what do you think
for these sorts of mojom interfaces with including in the comment who the
typical caller will be? Like, "Used by sysui to push the shelf state to...."

[msw, 2016/04/11 22:20:27]

I took both suggested comment changes even though (1) the wm ultimately owns the
shelf bounds and (2) I'm not sure if mentioning the current client is helpful.

+interface ShelfLayout {
+  SetAlignment(ShelfAlignment alignment);
+  SetAutoHideBehavior(ShelfAutoHideBehavior auto_hide);
+};