mash: Create and show a shelf in mash.

ash/common/shelf/shelf_window_watcher.cc

Index: ash/common/shelf/shelf_window_watcher.cc
diff --git a/ash/common/shelf/shelf_window_watcher.cc b/ash/common/shelf/shelf_window_watcher.cc
index 3288f955e6277d48658f5efad10fb9ee8e0c35a4..5470399e49d0b3f84a6754d5ffe2110379f59ac4 100644
--- a/ash/common/shelf/shelf_window_watcher.cc
+++ b/ash/common/shelf/shelf_window_watcher.cc
@@ -102,7 +102,8 @@ ShelfWindowWatcher::ShelfWindowWatcher(ShelfModel* model)
 
 ShelfWindowWatcher::~ShelfWindowWatcher() {
   display::Screen::GetScreen()->RemoveObserver(this);
-  WmShell::Get()->RemoveActivationObserver(this);
+  if (WmShell::HasInstance())

[James Cook, 2016/08/17 03:29:39]

Why is this needed? ShelfWindowWatcher is owned by WmShell and reset in
WmShell::Shutdown(), so I would expect there to always be a WmShell instance
around.

[msw, 2016/08/17 17:45:44]

Fixed instead by calling WmShell::Shutdown in WindowManager; thanks!
(seems like we should have been doing that all along)

+    WmShell::Get()->RemoveActivationObserver(this);
 }
 
 void ShelfWindowWatcher::AddShelfItem(WmWindow* window) {