Introduce mash/shell

mash/wm/background_layout.cc

Index: mash/wm/background_layout.cc
diff --git a/mash/wm/background_layout.cc b/mash/wm/background_layout.cc
index da6e1939d15442f43fb2cd9a8d3f44813904b65e..93c085a990fd269280672181022beafba3be3001 100644
--- a/mash/wm/background_layout.cc
+++ b/mash/wm/background_layout.cc
@@ -12,9 +12,9 @@ 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()));