Support additional mash shelf alignments.

mash/wm/public/interfaces/shelf_layout.mojom

Index: mash/wm/public/interfaces/shelf_layout.mojom
diff --git a/mash/wm/public/interfaces/shelf_layout.mojom b/mash/wm/public/interfaces/shelf_layout.mojom
new file mode 100644
index 0000000000000000000000000000000000000000..6ea69d703994e32d8ddc8587bc148083cb8679d9
--- /dev/null
+++ b/mash/wm/public/interfaces/shelf_layout.mojom
@@ -0,0 +1,18 @@
+// 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, };

[msw, 2016/04/12 00:14:00]

These enums already exist (in mash/shelf/public/interfaces/shelf.mojom and
ash/shelf/shelf_types.h), and shelf.mojom's Shelf interface is a more
feature-full version of the ShelfLayout interface below. Should I somehow share
these enum and/or interface definitions? Can we do so without having the
definitions live in mash/wm? (seems like they should live in mash/shelf, but
mash/shelf depends on mash/wm, not the inverse).

[sky, 2016/04/13 23:44:11]

The ash directory is likely to morph into the sysui directory. Seems like ash
should pull the enums from here. I recommend moving ShelfAlignment (and others
you want to sure) into a shelf_constants mojom that both places pull from.

[msw, 2016/04/15 17:25:06]

Added mash/shelf/.../shelf_constants.mojom, lmk if you wanted mash/wm...

+
+// 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);
+};