mash: Convert shelf to wm::WmWindow types, part 1

ash/shell/window_watcher.cc

Index: ash/shell/window_watcher.cc
diff --git a/ash/shell/window_watcher.cc b/ash/shell/window_watcher.cc
index 121a2c9f89289ecc0ce0a262db8b38c9b4bc9399..f171c34ec3de3eb4c7e3b1a8579908777f615015 100644
--- a/ash/shell/window_watcher.cc
+++ b/ash/shell/window_watcher.cc
@@ -45,7 +45,7 @@ class WindowWatcher::WorkspaceWindowWatcher : public aura::WindowObserver {
     panel_container->AddObserver(watcher_);
 
     aura::Window* container =
-        Shelf::ForWindow(root)->shelf_widget()->window_container();
+        ash::Shell::GetContainer(root, kShellWindowId_ShelfContainer);
     container->AddObserver(this);
     for (size_t i = 0; i < container->children().size(); ++i)
       container->children()[i]->AddObserver(watcher_);
@@ -57,7 +57,7 @@ class WindowWatcher::WorkspaceWindowWatcher : public aura::WindowObserver {
     panel_container->RemoveObserver(watcher_);
 
     aura::Window* container =
-        Shelf::ForWindow(root)->shelf_widget()->window_container();
+        ash::Shell::GetContainer(root, kShellWindowId_ShelfContainer);
     container->RemoveObserver(this);
     for (size_t i = 0; i < container->children().size(); ++i)
       container->children()[i]->RemoveObserver(watcher_);