mash: Convert all of //ash/system to use WmShelf

ash/common/shelf/wm_shelf.cc

Index: ash/common/shelf/wm_shelf.cc
diff --git a/ash/common/shelf/wm_shelf.cc b/ash/common/shelf/wm_shelf.cc
new file mode 100644
index 0000000000000000000000000000000000000000..d7831148afb6daa9732acd18e1e9d8dfd38bd5d4
--- /dev/null
+++ b/ash/common/shelf/wm_shelf.cc
@@ -0,0 +1,21 @@
+// 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.
+
+#include "ash/common/shelf/wm_shelf.h"
+
+#include "ash/common/wm_root_window_controller.h"
+#include "ash/common/wm_shell.h"
+#include "ash/common/wm_window.h"
+
+namespace ash {
+
+// static
+WmShelf* WmShelf::ForPrimaryDisplay() {
+  return WmShell::Get()
+      ->GetPrimaryRootWindow()
+      ->GetRootWindowController()
+      ->GetShelf();
+}
+
+}  // namespace ash