Introduce mash/shell

mash/wm/background_layout.cc

 // Copyright 2015 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 "mash/wm/background_layout.h"
 
 #include "components/mus/public/cpp/window.h"
 
 namespace mash {
 namespace wm {
 
 BackgroundLayout::BackgroundLayout(mus::Window* owner) : LayoutManager(owner) {}
 BackgroundLayout::~BackgroundLayout() {}
 
-void BackgroundLayout::WindowAdded(mus::Window* window) {
-  DCHECK_EQ(owner()->children().size(), 1U);
-}
+// We explicitly don't make assertions about the number of children in this
+// layout as the number of children can vary when the application providing the
+// background restarts.
 
 void BackgroundLayout::LayoutWindow(mus::Window* window) {
   window->SetBounds(gfx::Rect(owner()->bounds().size()));
 }
 
 }  // namespace wm
 }  // namespace mash